Updating legacy doc

[LikeTheSalad]

Temporary solution

Alternative to avoid old CI checks issues: #444

[colleenmcginnis]

I hope it's ok that I pushed a commit (70c7b8b). I tried to view the PR preview, but it said there were no changes so I pulled down this branch locally to build it. When I did that, I noticed a couple things:

1. There were several images missing from this branch resulting in warnings (that would likely become errors when published to the site) so I copied them over from main.

   INFO:build_docs:asciidoctor: WARNING: intro.asciidoc: line 27: can't read image [images/intro/distributed-tracing.png] at any of ["/doc/apm-agent-android/docs/images/intro/distributed-tracing.png", "/docs_build/resources/asciidoc-8.6.8/images/intro/distributed-tracing.png", "/doc/apm-agent-android/docs/images/images/intro/distributed-tracing.png"]
   INFO:build_docs:asciidoctor: WARNING: getting-started.asciidoc: line 127: can't read image [images/span-visualization/1.png] at any of ["/doc/apm-agent-android/docs/images/span-visualization/1.png", "/docs_build/resources/asciidoc-8.6.8/images/span-visualization/1.png", "/doc/apm-agent-android/docs/images/images/span-visualization/1.png"]
   INFO:build_docs:asciidoctor: WARNING: getting-started.asciidoc: line 132: can't read image [images/span-visualization/2.png] at any of ["/doc/apm-agent-android/docs/images/span-visualization/2.png", "/docs_build/resources/asciidoc-8.6.8/images/span-visualization/2.png", "/doc/apm-agent-android/docs/images/images/span-visualization/2.png"]
   INFO:build_docs:asciidoctor: WARNING: getting-started.asciidoc: line 136: can't read image [images/span-visualization/3.png] at any of ["/doc/apm-agent-android/docs/images/span-visualization/3.png", "/docs_build/resources/asciidoc-8.6.8/images/span-visualization/3.png", "/doc/apm-agent-android/docs/images/images/span-visualization/3.png"]
   INFO:build_docs:asciidoctor: WARNING: how-tos.asciidoc: line 91: can't read image [images/find-export-endpoint/1.png] at any of ["/doc/apm-agent-android/docs/images/find-export-endpoint/1.png", "/docs_build/resources/asciidoc-8.6.8/images/find-export-endpoint/1.png", "/doc/apm-agent-android/docs/images/images/find-export-endpoint/1.png"]
   INFO:build_docs:asciidoctor: WARNING: how-tos.asciidoc: line 95: can't read image [images/find-export-endpoint/2.png] at any of ["/doc/apm-agent-android/docs/images/find-export-endpoint/2.png", "/docs_build/resources/asciidoc-8.6.8/images/find-export-endpoint/2.png", "/doc/apm-agent-android/docs/images/images/find-export-endpoint/2.png"]
   INFO:build_docs:asciidoctor: WARNING: how-tos.asciidoc: line 100: can't read image [images/find-export-endpoint/3.png] at any of ["/doc/apm-agent-android/docs/images/find-export-endpoint/3.png", "/docs_build/resources/asciidoc-8.6.8/images/find-export-endpoint/3.png", "/doc/apm-agent-android/docs/images/images/find-export-endpoint/3.png"]
   INFO:build_docs:asciidoctor: WARNING: how-tos.asciidoc: line 105: can't read image [images/find-export-endpoint/4.png] at any of ["/doc/apm-agent-android/docs/images/find-export-endpoint/4.png", "/docs_build/resources/asciidoc-8.6.8/images/find-export-endpoint/4.png", "/doc/apm-agent-android/docs/images/images/find-export-endpoint/4.png"]
   

2. The lack of [discrete] headings was causing the content that should be on one page to broken up across multiple pages. I added [discrete] to the subheadings in each individual file. (In the screenshot below, every item is its own page.)

   Before	After

3. There were no defined IDs for each page so the resulting URLs were generated:

   Before	After
   https://www.elastic.co/guide/en/apm/agent/android/current/_introduction.html	https://www.elastic.co/guide/en/apm/agent/android/current/intro.html
   https://www.elastic.co/guide/en/apm/agent/android/current/_getting_started.html	https://www.elastic.co/guide/en/apm/agent/android/current/setup.html
   https://www.elastic.co/guide/en/apm/agent/android/current/_configuration.html	https://www.elastic.co/guide/en/apm/agent/android/current/configuration.html
   https://www.elastic.co/guide/en/apm/agent/android/current/_manual_instrumentation.html	https://www.elastic.co/guide/en/apm/agent/android/current/manual-instrumentation.html
   https://www.elastic.co/guide/en/apm/agent/android/current/_automatic_instrumentation.html	https://www.elastic.co/guide/en/apm/agent/android/current/automatic-instrumentation.html
   https://www.elastic.co/guide/en/apm/agent/android/current/_frequently_asked_questions.html	https://www.elastic.co/guide/en/apm/agent/android/current/faq.html
   https://www.elastic.co/guide/en/apm/agent/android/current/_howtos.html	https://www.elastic.co/guide/en/apm/agent/android/current/how-tos.html
   https://www.elastic.co/guide/en/apm/agent/android/current/_troubleshooting.html	https://www.elastic.co/guide/en/apm/agent/android/current/troubleshooting.html

[colleenmcginnis]

I also had to add a release notes page (97299d1) to avoid broken links across the AsciiDoc docs system (build). I couldn't find any release notes or changelog entries to add to the page. You'll have to add release notes when you're ready.

Here's the PR preview: https://apm-agent-android_bk_443.docs-preview.app.elstc.co/guide/en/apm/agent/android/current/intro.html

[LikeTheSalad]

Thank you so much, @colleenmcginnis 🙏 your changes saved me a lot of time, also thank you for your detailed description of them!