Improve top-level documentation accuracy and version context

[johnjasa]

Improve top-level documentation accuracy and version context

1. Align the README with the model overview and remove unsupported capability claims from the intro.
2. Update contributor, installation, and environment-variable guidance to match current project behavior.
3. Remove the obsolete unimplemented-features page and revise the H2Integrate design rationale.
4. Correct Read the Docs metadata, add a latest/stable version selector, and add a changelog page.

Section 1: Type of Contribution

• Documentation Update

Section 2: Draft PR Checklist

• Open draft PR
• Describe the feature that will be added
• Fill out TODO list steps
• Describe requested feedback from reviewers on draft PR
• Complete Section 8: New Model Checklist (if applicable)

Section 3: General PR Checklist

• PR description thoroughly describes the documentation changes.
• Added tests for new functionality or bug fixes
• Tests pass. The documentation build and configured pre-commit checks passed.
• Documentation
  • Docstrings are up-to-date (not applicable to these documentation changes)
  • Related docs/ files are up-to-date.
  • Documentation has been rebuilt successfully.
  • Examples have been updated (not applicable)
• CHANGELOG.md
  • At least one complete sentence has been provided to describe the changes made in this PR
  • A PR hyperlink is included as PR TBD pending the PR number.

Section 4: Related Issues

N/A

Section 5: Impacted Areas of the Software

Section 5.1: New Files

• docs/_static/switcher.json
  • Lists the Read the Docs latest and stable documentation versions.
• h2integrate/core/docs_sphinx.py
  • Sets the active Read the Docs version and displays the installed H2Integrate package version.

Section 5.2: Modified Files

• README.md
  • Links to the model overview instead of repeating an inaccurate technology list.
• docs/_config.yml, docs/_toc.yml
  • Corrects Read the Docs metadata and adds the version selector and changelog page to the book.

Section 6: Additional Supporting Information

The documentation version selector distinguishes the Read the Docs latest and stable builds.
The local build defaults to latest; the site announcement reports the installed H2Integrate
package version. The new changelog page includes the repository changelog.

Section 7: Test Results, if applicable

• bash docs/build_book.sh: passed with the documentation build configured to treat Sphinx warnings as errors.
• Targeted pre-commit checks on changed files: passed, including YAML, JSON, Ruff, and whitespace checks.
• git diff --check: passed.
• No runtime pytest tests were run. The Sphinx build exercised the documentation extension and pages.

Section 8 (Optional): New Model Checklist

N/A

[coveralls]

Coverage is 81.071% — update_top_docs into docs_guardrails. No base build found for docs_guardrails.