Docs(logo): Use MCP artwork in Sphinx (#134)

why: Sphinx needs the MCP mark at its existing docs asset paths, and
browser icons and link previews should identify the same application.

what:
- Replace the sidebar logo, favicon, and all existing manifest icons
- Add browser, mobile, Open Graph, and Twitter artwork under docs/_static
- Link the manifest and social cards from the Sphinx template
- Reuse the MCP PNG in the README and repository social preview

Verified with a clean Sphinx build, generated-link and package audits,
and Chromium, Firefox, and WebKit in light and dark themes. All six
Python/tmux CI cells pass at the merged PR head.

Author: tony

.github/social-preview.png

@@ -1,5 +1,14 @@
 # libtmux-mcp
 
+<!-- libtmux-logo -->
+<p align="center">
+  <picture>
+    <source srcset="docs/_static/img/libtmux.svg" type="image/svg+xml">
+    <img src="https://raw.githubusercontent.com/tmux-python/libtmux-mcp/main/docs/_static/img/libtmux.png" width="128" height="128" alt="libtmux for Python MCP">
+  </picture>
+</p>
+<!-- /libtmux-logo -->
+
 A [Model Context Protocol](https://modelcontextprotocol.io) server for [tmux](https://github.com/tmux/tmux), built on [libtmux](https://libtmux.git-pull.com).
 
 [![Python Version](https://img.shields.io/pypi/pyversions/libtmux-mcp.svg)](https://pypi.org/project/libtmux-mcp/)

README.md

@@ -0,0 +1,16 @@
+# Logo assets
+
+The documentation and README use the Python palette and MCP overlay from
+the libtmux asset kit. `libtmux.svg` is the vector mark; `libtmux.png` is
+its transparent 512px fallback. Both sidebar themes use the same mark.
+
+`icons/` contains the browser and mobile icons listed in
+`../../manifest.json`, Apple touch artwork, and the Safari mask.
+`../favicon.ico` contains the ICO fallback. `opengraph.png` is the
+1200 × 630 social card; `twitter.png` is the 1200 × 600 card. The repository
+social preview at `../../../.github/social-preview.png` uses the same MCP
+artwork as `opengraph.png`.
+
+The geometry comes from the project owner’s libtmux and tmuxp SVGs. The palette
+uses [GitHub Linguist’s Python color](https://github.com/github-linguist/linguist/blob/538da05f034fa5c83bd81df128d144e208898da3/lib/linguist/languages.yml).
+The overlay uses the symbol from the [MCP logo](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/088176704a4e8f20e7f1d237de1de8484aa412fd/docs/logo/light.svg).