feat: include gh-admonitions by default

[stephenglass]

Summary

Adds gh-admonitions as a default bundled MkDocs plugin in mkdocs-techdocs-core.

This enables GitHub alert syntax such as > [!NOTE], > [!TIP], > [!IMPORTANT], > [!WARNING], and > [!CAUTION] to render as MkDocs Material admonitions without requiring TechDocs users to configure the plugin separately.

Changes

• Added mkdocs-github-admonitions-plugin==0.1.1 as a pinned dependency.
• Registered the gh-admonitions plugin by default from techdocs-core.
• Added unit coverage confirming the plugin is injected into the MkDocs plugin config.
• Updated the README plugin list to document the new bundled plugin.

Testing

• python -m unittest techdocs_core/test_core.py
• pip install --editable .
• mkdocs build --config-file test-fixtures/mkdocs.yml
• black --check .