Publish Markdown and llms.txt docs with ExDoc 0.40.3

[BobbieBarker]

Important

This PR is stacked on #243, which makes the required Elixir 1.15 compatibility change explicit. Please review and merge #243 first; once it lands, this PR's effective diff is only the ExDoc dependency and lockfile update.

Summary

• require ExDoc 0.40.3 for development documentation
• update the locked ExDoc documentation toolchain
• preserve Decimal's existing HTML and EPUB documentation while adding native Markdown pages and llms.txt

Why this matters

ExDoc v0.40 added native Markdown output and llms.txt generation. Publishing those artifacts on HexDocs makes Decimal's documentation directly consumable by modern documentation retrieval tools, including docs-mcp-server, without scraping the HTML site. Broad library adoption is what makes this useful across the Elixir ecosystem.

After this merges, please publish or re-publish Decimal's documentation at your earliest convenience so the Markdown pages and llms.txt become available to the community.

Compatibility

ExDoc remains a development-only dependency and does not become part of Decimal's runtime dependency graph. The ExDoc release itself requires Elixir 1.15, which is why the runtime compatibility decision is isolated in #243.

Verification

• unmodified baseline: MIX_ENV=dev mix docs generated HTML and EPUB, with no llms.txt or Markdown pages
• changed build: MIX_ENV=dev mix docs generated HTML, EPUB, llms.txt, and 5 Markdown files
• all 4 links emitted in llms.txt resolve to generated files
• mix format --check-formatted mix.exs
• MIX_ENV=test mix deps.unlock --check-unused
• MIX_ENV=test mix compile --warnings-as-errors
• MIX_ENV=test mix test — 266 passed (105 doctests, 36 properties, 125 tests)