feat: add r/docs/home

[moul]

Introducing the r/docs namespace, where the homepage currently lists subrealms manually. In the future, we may implement a registry, but for now, we’re keeping the source code as lean as possible. The namespace includes several interactive examples to guide users through key concepts.

The r/docs/hello example provides a simple Render function and invites users to click on "view source" to understand the basics of customization. The r/docs/avl_pager example demonstrates path-based interactions, allowing users to explore an avl tree structure with pagination links to navigate between items. Users are encouraged to click on these links for inspiration before manually adjusting parameters in the URL. The added r/docs/add example introduces interactivity through transactions, allowing users to adjust a number by submitting transactions, and see the updated result with each interaction.

These examples are designed to engage users with Render-based UI interactions, path handling, and transaction-based updates. Once we have more content in r/docs, this section could serve as the main documentation link in the navbar, providing a comprehensive, hands-on introduction to Gno.

Addresses #3084
Addresses https://github.com/gnolang/docs-v2/pull/27#discussion_r1848481556
Addresses #2953

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.79%. Comparing base (b3800b7) to head (ba3ff33).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3160      +/-   ##
==========================================
- Coverage   63.80%   63.79%   -0.01%     
==========================================
  Files         549      549              
  Lines       78833    78833              
==========================================
- Hits        50301    50294       -7     
- Misses      25143    25149       +6     
- Partials     3389     3390       +1     

Flag	Coverage Δ
contribs/gnodev	61.16% <ø> (ø)
contribs/gnofaucet	15.77% <ø> (+0.94%)	⬆️
gno.land	73.70% <ø> (ø)
gnovm	67.94% <ø> (ø)
misc/genstd	79.72% <ø> (ø)
tm2	62.27% <ø> (-0.11%)	⬇️

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[leohhhn]

We should have a "root" README for the demo, sys, gov, docs namespaces. This would be displayed when users look up the namespace root, ie gno.land/r/docs/, apart from some other info, like the list of pkgs, number, etc (tbd with gnoweb2).

I can help bootstrap these files if needed. They're meant to give context to the users about what the namespace is for.

Also, all/most r/docs realms would correspond to a specific package, concept, usage; we should reference those when it makes sense; maybe as a doc.gno giving a brief overview apart from the code itself.