docs(adapters): remove html_to_markdown post-processor from selectors reference

[gildesmarais]

What changed

• Remove the html_to_markdown bullet from src/content/docs/ruby-gem/reference/selectors.mdx (post-processor reference).
• Keep markdown_to_html and sanitize_html documented.

Why

Gem PR html2rss/html2rss#500 drops the html_to_markdown post-processor and reverse_markdown dependency as an intentional breaking schema change. Public docs must match the published post-processor list.

Risk

• Low — documentation-only; no runtime behavior in this repo.
• Merge after (or with) the gem PR so the reference does not lag the schema.

Review map

Review map: whole PR is small — start at src/content/docs/ruby-gem/reference/selectors.mdx.

Validation

• make check and make build green on docs/drop-html-to-markdown.

Test plan

• CI / make check on this PR
• make build succeeds
• Selectors reference lists only supported post-processors (no html_to_markdown)

Related

• Gem: chore(deps): drop reverse_markdown and html_to_markdown post-processor html2rss#500