Add markdown formatter to docs output

[oliver-kriska]

ex_doc v0.40.0 (January 2026) added a Markdown formatter and generates llms.txt by default. Because this project pins an explicit formatters: list, the new default never applies, so the docs build correctly but are not retrievable as markdown.

This adds "markdown" to the existing list. The HTML output is unchanged, and no other formatter is added or removed.

After the next docs publish, hexdocs.pm/x509/llms.txt will start resolving and individual pages will become available as .md.

Verification:

• mix docs passes and emits doc/llms.txt plus Markdown pages.
• mix format --check-formatted passes.
• mix compile --warnings-as-errors passes.
• mix test --include openssl has one pre-existing, date-sensitive failure: the EdDSA certificate fixture is expired ({:bad_cert, :cert_expired} at test/x509/certificate_test.exs:218). The same targeted test fails unchanged on the unmodified upstream branch.

This change was generated from Elixir's retrieval gap is a rollout problem, not an access problem.

Note: this package's currently published docs were built with an older ex_doc, so the markdown output will appear after the next mix hex.publish docs — no rush, just flagging that the merge alone won't change hexdocs.

— Oliver’s AMP