Initial sidebar links, further setup tweaks

[jodyheavener]

Gist: this PR adds a bunch of empty doc pages and external links for new sections in the sidebar, a basic Welcome doc, a handful of configuration changes, search capability, and some cosmetic tweaks.

Specifically:

• Adds a Welcome page detailing how to contribute from the UI
• Adds sidebar sections and external links according to the issue
  • This ticket says to create a "Mana" section, but I took a slightly different approach because lots of pages link to Mana and so instead of shoehorning them into that one section I tried breaking them out into their own, as well as adding Mana to the top navbar.
• Adds some more sidebar links according to the "Existing docs" section of the Jira Epic
• Removes the home page entirely, putting the app into a "Docs-only mode", and setting the Welcome doc as the home page (now it can also be managed with Markdown!)
• Disables the dark/light mode switch, instead using the system preference
• Allow sidebar to be hide-able
• Adds Nimbus link to top navbar
• Some cosmetic changes
  • Use GitHub icon in top navbar
  • Update Favicon to be visible in light/dark browsers
  • Add dark mode logo
  • Style external sidebar links with an icon
• Adds Algolia search

[jodyheavener]

Before I put this up, @clouserw suggested that it might be nice to model the sidebar links using this guidance. I like the idea of having tutorials, guides, and references, and this kind of setup could help keep things organized. Since this is a hub and there will be sections that only apply to certain groups of people (i.e. data science might not care about desktop engineer stuff), maybe these organizing categories could live underneath the links of each topic. cc @LZoog @k88hudson thoughts?

[LZoog]

I like the idea of having tutorials, guides, and references, and this kind of setup could help keep things organized.

I like the way this looks as well, though is it a little premature to make it this layered with the amount of docs that we have planned right now? Maybe we could create a follow up ticket for this? It looks great how you have it in this PR IMO with the collapsible sections. :)

[LZoog]

This LGTM! I double checked all the links were there, and I think this setup/organization is great👌

[LZoog]

Wellllll... I could have sworn Docusaurus did this automagically. ☹️ I'll figure this out when working on the maintenance workflow docs (you've already written like more than half of it so thanks, ha).

[jodyheavener]

I like the way this looks as well, though is it a little premature to make it this layered with the amount of docs that we have planned right now? Maybe we could create a follow up ticket for this? It looks great how you have it in this PR IMO with the collapsible sections. :)

Thanks for your input here! I do like the suggested way of organizing into references, guides, tutorials, etc. but you might be right that it's early for this. I've created https://jira.mozilla.com/browse/EXP-1048 so we can follow up and maybe try a re-organization once we have more content. cc @clouserw