Allow selection of headlines

[watson]

Today it's not possible to select headlines with your mouse so that you can copy it. This is important for the APM Node.js agent as the headlines in many cases are command that the user needs to type, so not being able to select and copy them is really annoying.

Here's an example of one such headline:

https://www.elastic.co/guide/en/apm/agent/nodejs/current/agent-api.html#error-on-aborted-requests

[watson]

ping @elastic/docs

[bmorelli25]

I've noticed this in other documentation books as well. Here are some examples.

• Beats documentation: https://www.elastic.co/guide/en/beats/heartbeat/current/configuration-logging.html#_literal_logging_to_stderr_literal
• APM Node.js Agent:
  https://www.elastic.co/guide/en/apm/agent/nodejs/current/agent-api.html
• Logstash filter plugins: https://www.elastic.co/guide/en/logstash/current/plugins-filters-alter.html#plugins-filters-alter-coalesce

cc @nik9000

[nik9000]

I'll take a look, sure.

[nik9000]

This happens because the <a> tag that makes the little link selector show up covers up the title text. I'll see if I can undo that.