Integrate website redesign: `david-allison/ankidroid-website`

We have a production-ready landing page (and planned site) located at: https://github.com/david-allison/ankidroid-website

This issue tracks tasks which are required before we go live.

## Decisions

- This repo will be the source of truth for publishing to https://ankidroid.org, containing data currently in `ankidroid-website`
- The manual will be served by https://docs.ankidroid.org
- Merge into this repo. Do not use `git subtree`

## Current state

- Both https://docs.ankidroid.org and https://ankidroid.org serve the manual. https://docs.ankidroid.org is a CNAME to https://ankidroid.org
- This repo is deployed via https://github.com/ankidroid/ankidroiddocs/blob/main/.github/workflows/publish.yml
- docs are located at `ankidroid@ankidroid.org:/usr/share/nginx/html/`
- The website repo handles existing `#` redirects

## Tasks

- [ ] Evaluate options for merging `repo:ankidroid-website` into `repo:ankidroiddocs`
- merge the repo in 🏗️
- ideally in a `ankidroid.org` folder in this repo 🏗️
- move `ankidroid-website` CI to `ankidroiddocs` 🏗️
- Consider issues/PR discussions and linkage to the old repo
- we could also rebase commits to cross-link the repo/PRs
- [ ] Update CI to deploy a new folder/webroot (without nginx hosting it inside `/usr/share/nginx/html/`)
- ensure `index.html` is handled for both webroots.
- rethink `/usr/share/nginx/html/` usage; `/var/www/docs.ankidroid.org/` would likely be better
- [ ] Update nginx
- we could potentially do with nginx documentation in this repo - syncing this between the live site may be an issue
- fix redirects: `help.html`/`changelog.html` etc... need to be handled from the root

## Follow-ups

- [ ] Determine if we rename this repo