fix: fetch plugin documentation (#26)

fixes #23 . 

Since GH became more agressive with its rate limiting, the collection of
plugin docs would fail with a HTTP status code 429: Too many requests.

This PR moves the markdown fetching logic from plain http requests to
using gitPython.

## Test Plan
1. run `pixi run build`
2.  Open the `build/index.html` file and check that 
a. Plugins that provide 'intro.md' (e.g. the slurm executor plugin) have
their documentation displayed
b. Pages that do not provide their own documentation show a little
warning box


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Refactor**
* Switched plugin documentation retrieval to a Git-based cloning
approach with multi-branch support, replacing the prior HTTP-based
retrieval.

* **Chores**
* Added a Git-related dependency (gitpython) to support repository
cloning.

* **Style**
* Cleaned up CI workflow YAML formatting and alignment; no behavioral
changes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Lea Evers <l.evers@yandex.com>

Author: schnea

.github/workflows/deploy.yml

@@ -2,11 +2,11 @@ name: Deploy
 
 on:
   push:
-    branches: [ main ]
+    branches: [main]
   workflow_dispatch:
   schedule:
     # * is a special character in YAML so you have to quote this string
-    - cron:  '50 4 * * *'
+    - cron: "50 4 * * *"
 
 defaults:
   run:
@@ -16,31 +16,31 @@ jobs:
   deploy:
     # Grant GITHUB_TOKEN the permissions required to make a Pages deployment
     permissions:
-        pages: write      # to deploy to Pages
-        id-token: write   # to verify the deployment originates from an appropriate source
+      pages: write # to deploy to Pages
+      id-token: write # to verify the deployment originates from an appropriate source
     environment:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}
 
     # Specify runner + deployment step
     runs-on: ubuntu-latest
     steps:
-        - uses: actions/checkout@v4
+      - uses: actions/checkout@v4
 
-        - uses: prefix-dev/setup-pixi@v0.9.0
-          with:
-            cache: true
+      - uses: prefix-dev/setup-pixi@v0.9.0
+        with:
+          cache: true
 
-        - name: Building
-          run: pixi run build
+      - name: Building
+        run: pixi run build
 
-        - name: Setup Pages
-          uses: actions/configure-pages@v5
-        - name: Upload artifact
-          uses: actions/upload-pages-artifact@v3
-          with:
-            path: "build"
+      - name: Setup Pages
+        uses: actions/configure-pages@v5
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: "build"
 
-        - name: Deploy artifact
-          id: deployment
-          uses: actions/deploy-pages@v4
+      - name: Deploy artifact
+        id: deployment
+        uses: actions/deploy-pages@v4