ci: add vimdoc auto-generation workflow (#23)

vimdoc should update on every commit to `main`. This way the "Last change" info in vimdoc actually shows when the code was last changed even if the source file for help didn't change.

Other notable changes:
- Made README.md panvimdoc compatible
  - Got rid of `<details>` and `<summary>` tags in installation instructions
  - Made title, description and badges markdown only features (not included in vimdoc)
- Tweaked README.md appearance
  - Raised all heading levels by one
  - Updated description
  - Added license badge
  - Removed customisations from badges
    - Left style as For the Badge though
- Added initial vimdoc, locally generated with panvimdoc

Closes #10.

.github/workflows/docs.yml

@@ -0,0 +1,24 @@
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  docs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: kdheepak/panvimdoc@v3
+        with:
+          vimdoc: auto-save.nvim
+          version: "Neovim >= 0.8.0"
+          demojify: true
+          treesitter: true
+          shiftheadinglevelby: -1
+      - uses: stefanzweifel/git-auto-commit-action@v4
+        with:
+          branch: ${{ github.head_ref }}
+          commit_message: "chore(doc): auto-generate vimdoc"
+          commit_user_name: "github-actions[bot]"
+          commit_user_email: "github-actions[bot]@users.noreply.github.com"
+          commit_author: "github-actions[bot] <github-actions[bot]@users.noreply.github.com>"