docs(chore): Remove files now provided by sphinx-gptheme

why: All removed files are now bundled in sphinx-gptheme and loaded
     automatically, making local copies pure maintenance debt.
what:
- Delete argparse-highlight.css — identical to theme bundled version
- Delete custom.css — replaced by tmuxp.css (12-line project-specific
  aspect-ratio overrides only; all generic CSS now from theme)
- Delete _templates/page.html — identical to theme version; mask-icon
  moved to theme_options.mask_icon instead of hardcoded in template
- Delete _templates/sidebar/brand.html — identical to theme version
- Delete _templates/sidebar/projects.html — identical to theme version
- Add docs/_static/css/tmuxp.css with only tmuxp-specific image
  aspect-ratio rules (tmuxp-demo, tmuxp-shell, tmuxp-dev-screenshot)
- Update conf.py: css/custom.css → css/tmuxp.css,
  add theme_options={"mask_icon": "/_static/img/tmuxp.svg"}
- Update uv.lock to gp-sphinx init-2 commit c2fe249
  (theme: move mask-icon outside show_meta_app_icon_tags guard)

Author: tony

docs/_static/css/argparse-highlight.css

@@ -0,0 +1,12 @@
+/* Per-image aspect ratios for CLS reservation before load */
+img[src*="tmuxp-demo"] {
+  aspect-ratio: 888 / 589;
+}
+
+img[src*="tmuxp-shell"] {
+  aspect-ratio: 878 / 109;
+}
+
+img[src*="tmuxp-dev-screenshot"] {
+  aspect-ratio: 1030 / 605;
+}