Simplify what it takes to write module docs

[srid]

The README has this footnote,

In particular, we want to obviate the user from having to do this (using https://github.com/srid/haskell-flake/tree/master/doc as example):

1. [x] haskell-flake.md should just be index.md (see https://github.com/srid/emanote/issues/494); and haskell-flake/*.md should all be moved to top-level #18
2. [ ] In the index markdown we have this YAML meta, that should not be needed either:
3. [ ] A convenient nix run ... command to preview local module docs with the rest of the site using standard flake inputs (for other module docs).
   • [x] Part I: https://github.com/flake-parts/community.flake.parts/pull/52
   • [ ] Part II: Can we avoid needing a ./doc/flake.nix?
4. [ ] Checks
   • [ ] Check for dangling nodes in flake check (as part of build), to avoid cases like https://github.com/juspay/services-flake/pull/150
   • [ ] Check for flat slugs, https://github.com/juspay/services-flake/pull/153

[srid]

The layout part is fixed; now we need to address the extraneous YAML frontmatter part.