📝 docs: restructure to follow Diataxis framework

[gaborbernat]

The virtualenv documentation was organized as a flat collection of feature-oriented pages (user_guide.rst, cli_interface.rst, installation.rst, extend.rst), making it hard for users to find what they need depending on whether they're learning, doing a task, or looking up a reference. 📚 New contributors and plugin authors had no clear starting point, and several features like PEP 440 version specifiers, free-threading support, and programmatic activation via activate_this.py were undocumented.

This restructures the entire documentation following the Diataxis framework into four quadrants — tutorials, how-to guides, reference, and explanation — plus a dedicated plugin section that itself follows the same structure. A new getting-started tutorial walks users through creating, activating, installing packages, and reproducing environments step by step. How-to guides cover installation (with uv/pipx/pip/zipapp tabs), usage patterns, and Python version selection. The reference section provides CLI flags via the existing table_cli directive, autodoc API docs, and a compatibility matrix. A single explanation page covers how virtualenv works, discovery, creators, seeders, and activators with mermaid diagrams. The plugin section adds a tutorial, task-oriented guides, API reference with autodoc, and an architecture explanation.

On the tooling side, the Sphinx theme moves to Furo with the virtualenv logo (recolored to Python blue/yellow) as the sidebar icon and SVG favicon. ✨ Extensions now include sphinx-inline-tabs for platform-specific instructions, sphinxcontrib-mermaid for diagrams, sphinx-copybutton for code blocks, and sphinxcontrib-towncrier replacing the manual changelog draft generation. A docstrfmt pre-commit hook enforces consistent RST formatting. All public plugin API classes (Discover, Creator, Seeder, Activator, AppData, PythonInfo, Session, VirtualEnvOptions) now carry full type annotations with Sphinx :param:/:returns: docstrings, enabling proper intersphinx cross-linking in the rendered docs. The ty type checker passes with zero diagnostics. Furo's search input is wired to Read the Docs Addons' search-as-you-type modal for the production site.

Old pages (user_guide.rst, cli_interface.rst, installation.rst, extend.rst) are removed — all their content has been redistributed into the new structure. The changelog switches from .. include:: _draft.rst to the towncrier-draft-entries directive. Mermaid diagram colors use a dual-theme palette (amber, blue, green, indigo, purple) that works on both light and dark backgrounds.