docs(format): add FM-Index to scalar index navigation

[jackylee-ch]

fmindex.md has been in the tree since #7123 but was never added to the
scalar index nav, so the page is published yet unreachable from the sidebar:
lance.org/format/index/scalar/fmindex/ returns 200, while the BTree page
lists only its seven other siblings.

Placed between N-gram and RTree, matching the file's position in the
directory listing.

Documentation only; no format change.

[github-actions]

Important

This PR touches the Lance format specification.

Substantive changes to the format specification — the .proto definitions
and the spec docs under docs/src/format/ — require a PMC vote before merge.
Minor edits such as typo fixes, wording, or formatting are excluded; use your
judgment.

If this is a meaningful format change:

• Start a vote following the Lance community voting process.
  Format specification modifications need 3 binding +1 votes (excluding the
  proposer), held on GitHub Discussions, with a minimum voting period of 1 week.
• Once the vote passes, link the completed vote in this PR. It should not be
  merged until the vote is linked.

[lance-gatekeeper]

✅ Gate recommendation: approve.

This restores sidebar discoverability for the existing FM-Index format page without changing the format contract.

[westonpace]

I'm going to go ahead and say this is not a change to the spec. This is just restoring docs that were missing.