Add API documentation to README

[joewiz]

Closes #2.

Summary

Adds an API Documentation section to the README covering all public variables and functions in semver.xqm, generated using xquerydoc.

The section includes:

• A table of contents linking to every variable and function
• Signature, description, params, return type, and errors for each entry
• Anchor links for direct deep-linking to individual functions

How it was generated

# From the xquerydoc repo:
java -Xss16m -cp "target/dependency/*" net.sf.saxon.Query \
  -q:- "semverpath=file:///path/to/semver.xq/src/main/xquery/semver.xqm" \
  <<< '...' > semver-xqdoc.xml

java -cp "target/dependency/*" net.sf.saxon.Transform \
  -s:semver-xqdoc.xml \
  -xsl:src/lib/markdown-module.xsl \
  -o:semver-api.md

The output was lightly reformatted for better GitHub Markdown rendering (bold param/return labels, inline signatures).

🤖 Generated with Claude Code