Add sidebar, navigation, and/or property search

[rphair]

This issue might change in scope with further time & contributions. Generally: the recent eradication of CIP content long rendered from GitHub source into the Developer Portal:

• Proposal to remove CIPs from the developer portal developer-portal#1362
• Combined upgrade: Docusaurus 3.6, remove CIP &, Changelog, add Author and Tag files developer-portal#1367

has left users with less navigation than they are used to (e.g. cardano-foundation/developer-portal#1362 (comment)). I think we should consider the following possibilities for a sidebar:

• a big bar, with all the CIPs listed in numerical order
• some smaller bars with search facets by CIP metadata fields

This also leads directly to the question of what "search facets" might be like, considering that search/sort by metadata fields {category, author, status, ...} isn't possible even on the main listing page.

I don't know how to refine these suggestions (short of contributing a PR, which would be beyond the scope of my own development bandwidth or skills now) because I don't know who's maintaining cips.cardano.org, what their availability might be like, what commitment they might have to support community requests, etc.

Considering that this is now the main source for CIP content rendered from GitHub, I think we first need some advice about the latter support questions, to help set community expectations (cc @fill-the-fill @aldo555 @KtorZ). 🙏