Serve clean Markdown versions of every documentation page

[Copilot]

Documentation pages are only served as HTML, which is hostile to AI ingestion. This PR exposes a clean .md version of every page, plus a manifest and a content-negotiation worker.

Markdown export plugin

• _plugins/markdown_pages.rb writes a sibling .md file for every page during the Jekyll build (/foo/bar.html → /foo/bar.md, / → /index.md).
• Source is re-read from disk (Jekyll mutates page.content to HTML in-place by :site, :post_write), front matter stripped, then rendered through Liquid only — converters are skipped so output stays markdown.
• Skips layout: redirect pages and any page with markdown_skip: true in front matter.
• Emits /.well-known/markdown-index.txt listing every markdown URL for crawlers.

Clean output

• _includes/code_tabs.html checks a markdown_format flag set by the plugin and emits plain fenced code blocks instead of the radio/label tab widget. HTML rendering is unchanged.
• Layouts, nav and footer never appear because we render page.content directly, not through do_layout.

# GRAPH.QUERY

Executes the given query against a specified graph.

```python
graph.query("CREATE VECTOR INDEX FOR (p:Product) ON (p.description) ...")

### Build pipeline
- GitHub's built-in Pages builder runs Jekyll in safe mode and ignores `_plugins/`, so deployment moves to a GitHub Actions workflow (`.github/workflows/pages.yml`) using `actions/configure-pages` + `actions/deploy-pages`.
- Adds `Gemfile` (Jekyll 3.10, just-the-docs, kramdown-parser-gfm, the three existing plugins) and ignores `Gemfile.lock` / `vendor/` / `.jekyll-cache/`.

### `Accept: text/markdown` content negotiation
- Static Pages can't read request headers, so `markdown-content-negotiation.worker.js` is shipped as a reference Cloudflare Worker. It rewrites requests with `Accept: text/markdown` (or `text/x-markdown` / `text/plain`) to the corresponding `.md` URL, sets `Content-Type: text/markdown; charset=utf-8` and `Vary: Accept`, and falls back to the HTML response if the `.md` sibling 404s.

### Misc
- README documents the `.md` URL convention, the manifest path, and the worker.
- Adds `CDN`, `Cloudflare` to `.wordlist.txt` for spellcheck.

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by firewall rules:
>
> - `docs.falkordb.com`
>   - Triggering command: `/usr/bin/curl curl -sI REDACTED` (dns block)
>   - Triggering command: `/usr/bin/curl curl -sI -L REDACTED` (dns block)
>   - Triggering command: `/usr/bin/curl curl -v REDACTED` (dns block)
>
> If you need me to access, download, or install something from one of these locations, you can either:
>
> - Configure [Actions setup steps](https://gh.io/copilot/actions-setup-steps) to set up my environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this repository's [Copilot coding agent settings](https://github.com/FalkorDB/docs/settings/copilot/coding_agent) (admins only)
>
> </details>