Convert absolute links to document embedded objects to raw content refs

[g-pavlov]

What this PR does / why we need it:
Links to embedded objects, such as images on GitHub like: https://github.com/gardener/gardener/blob/v1.11.1/logo/gardener-large.png get redirected to https://raw.githubusercontent.com to get the raw content for the image to be embedded and that mostly doesn't work well with every non-gitHub environment. In addition documentation is written with all sorts of image reference links.
Now, every blob link that is resolved as absolute on a document, will be rewritten for 'raw' content type link that GitHub resolves correctly to requests for raw content.
Raw https://raw.githubusercontent.com links like also rewritten to the match the same consistent pattern.

The github.ResourceLocator object is now able to resolve URLs for every GitHub object - code (tree|blob), wiki (page), releases, pull(s), issue(s) and raw.githubusercontent links.

Processed destinations in document links are now inspected more carefully and returned early without modifications if they cannot/should not be processed.

Failures to parse a GitHub URL are now reported as errors.

Release note:

Absolute links to page embedded objects (images) in documents are now rewritten to match GitHub's format for raw content links consistently and display embedded objects correctly. The behavior is controlled with the `--rewrite-embedded-to-raw` flag, which is true by default.

Failures to parse a GitHub URL are now reported as errors. Invalid links on the documents will now fail the build.