Enable Markdown documentation with the latest ExDoc plugin

[danielvelara]

Summary

• Pin rebar3_ex_doc to its latest release, 0.3.0.
• Add rebar3 docs to generate HTML, Markdown, and EPUB documentation, including llms.txt.
• Use the same documentation alias for Hex publishing.
• Add a documentation check to the OTP 28 CI job.
• Document the generation and archive commands in RELEASE.md.

The latest Rebar3 plugin bundles ExDoc 0.40.3, which supports Markdown. ExDoc 0.40.4 is newer, but the plugin does not bundle it yet.

The plugin explicitly requests HTML and EPUB by default. The alias adds the Markdown formatter while preserving both existing formats. Its explicit default namespace also works when the Hex provider invokes it.

rebar.lock remains unchanged because the project has no application dependencies, and Rebar3 does not lock project plugins there.

Verification

With Rebar3 3.27.0:

• rebar3 compile: passed on OTP 27 and 28.
• rebar3 ct --cover: all 42 tests passed on OTP 27 and 28.
• rebar3 xref: passed on OTP 27 and 28.
• rebar3 dialyzer: passed on OTP 28.
• rebar3 docs: generated HTML, Markdown, and EPUB on OTP 28.
• rebar3 hex build docs: built the documentation archive on OTP 27 and 28.
• Verified all seven Markdown files, their HTML counterparts, and the module and extra-page entries in llms.txt.
• Verified that the Hex documentation archive includes the Markdown files, llms.txt, HTML, and EPUB.
• Parsed the workflow YAML and ran git diff --check.

Documentation generation still reports an existing invalid type link in CHANGELOG.md:32 (telemetry:span_function/0). The original configuration reports the same warning. This PR leaves that unrelated link unchanged.

[josevalim]

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