SDK links checker

Some SDKs have a non-trivial number of download links:

https://docs.couchbase.com/c-sdk/current/project-docs/sdk-release-notes.html#downloads

https://docs.couchbase.com/php-sdk/current/project-docs/sdk-release-notes.html#sdk-installation

Best effort manual checking needs replacing with scripted checking each time a release notes page is modified - a script to check that each link produces an artefact, not a 404 or 403.