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. haskell-flake.md should just be index.md (see Combining multiple notebooks srid/emanote#494); and haskell-flake/*.md should all be moved to top-level Simplification 1: index.md #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).
   • Part I: Add a docs flake module for re-use #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 docs: (readme) link to datadir doc from cfp; (index) add guide juspay/services-flake#150
   • Check for flat slugs, chore(doc): Slugs must be flat juspay/services-flake#153

[srid]

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