Add markdown formatter to docs output

[oliver-kriska]

ex_doc v0.40.0 added a Markdown formatter and generates llms.txt by default. Because this project pins an explicit formatter list that predates that release, the new default does not apply.

This adds only "markdown"; HTML output is unchanged. mix docs now emits doc/llms.txt and Markdown pages. The sibling geo_postgis formatter pin is being handled separately for consistency.

Note: the currently published docs are stale, so Markdown output will appear after the next mix hex.publish docs; merging alone will not change HexDocs.

Generated from the findings in Elixir's retrieval gap is a rollout problem, not an access problem.

— Oliver’s AMP