ci: add markdown, website, and image lint checks (#882)

Adds Markdown, website, and image lint checks to CI, mirroring the setup
in `apify-docs` and `apify-core`. The docs and website now go through
the same kind of automated checks as the Python code.

The website tooling moves from eslint + prettier (which weren't wired
into CI) to [oxlint](https://oxc.rs/) and [oxfmt](https://oxc.rs/), and
[markdownlint](https://github.com/DavidAnson/markdownlint) now covers
the Markdown files.

Three jobs run in the existing Checks workflow:

- **Markdown lint** — `markdownlint` over `README.md`,
`CONTRIBUTING.md`, and the `docs/` folder.
- **Website lint and format** — `oxlint` plus an `oxfmt --check`
formatting gate for the Docusaurus site.
- **Image lint** — fails when a PR adds unoptimized raster images, which
should be converted with `pnpm opt:images` first.

Notes:

- The website JS keeps single quotes, set via `.editorconfig`
(`quote_type`) and `.oxfmtrc.json` (`singleQuote`).
- The markdownlint config matches `apify-docs`: line length is disabled
and inline HTML is allowed.
- Minor doc and code fixes were applied where needed to pass the new
checks.

Closes #594