User guide: consolidate prerequisites into a single section or page

[chalin]

With the introduction of more than one installation option, we now have duplicate explanations of how to setup prerequisites. For example, compare:

• https://www.docsy.dev/docs/get-started/docsy-as-module/installation-prerequisites/
• https://www.docsy.dev/docs/get-started/other-options/#prerequisites

For those parts that apply to all installation options (which is most if not all parts), these should be pulled out into a separate Prerequisites section.

WDYT @LisaFC @emckean

[LisaFC]

Yes, I think that's a great idea (in fact I literally did the same thing recently for the docs for something else).

We can add a quick intro with "These prerequisites apply to all installation options..." etc. etc. Then each installation option guide can have its own "Before you begin" with:

• Ensure you have all the prerequisites as described in...
• [any option-specific requirements]

[chalin]

This is still relevant, in fact now some sections are becoming even more out-of-sync (e.g., #1597). The content of these sections aren't fully consistent:

• https://www.docsy.dev/docs/get-started/other-options/#prerequisites
• https://www.docsy.dev/docs/get-started/docsy-as-module/installation-prerequisites/