chore: update ExDoc to 0.40.3

[BobbieBarker]

Summary

• update the optional development-only ExDoc dependency from 0.37.3 to 0.40.3
• enable Markdown documentation and llms.txt generation through ExDoc's new default formatter set
• preserve ErrorMessage's existing HTML and EPUB documentation outputs

Why this matters

ExDoc 0.40 added native Markdown documentation and automatic llms.txt generation. Publishing these artifacts on HexDocs makes ErrorMessage's documentation easier to retrieve and use from modern Elixir development tools, documentation servers, and coding agents while preserving the existing human-readable documentation.

This contributes to machine-readable documentation coverage across the Elixir ecosystem and helps community tooling provide accurate, version-specific guidance about ErrorMessage.

After merging, please publish or republish ErrorMessage's package documentation at your earliest convenience so the new artifacts become available through HexDocs and can support the broader community.

Compatibility

• ErrorMessage's package requirement remains Elixir 1.7; this PR does not raise it
• ExDoc remains optional, development-only, and runtime: false, while the project's native CI runs in the test environment
• verified the full test suite on Elixir 1.14, below ExDoc 0.40's requirement, demonstrating that the documentation dependency does not change runtime compatibility
• there are no runtime dependency or public API changes

Verification

• unmodified baseline: MIX_ENV=dev mix docs with ExDoc 0.37.3 produced HTML and EPUB documentation with no llms.txt or Markdown pages
• changed build: MIX_ENV=dev mix docs with ExDoc 0.40.3 produced HTML, EPUB, llms.txt, and 12 Markdown pages
• verified that all 11 Markdown links in llms.txt resolve to generated files
• MIX_ENV=test mix test on Elixir 1.14.3 — 4 tests and 106 doctests, 0 failures
• MIX_ENV=test mix credo — no issues
• MIX_ENV=test mix dialyzer — 0 errors
• MIX_ENV=test mix coveralls.json — 4 tests and 106 doctests, 0 failures; 65.7% total coverage