Build: environment variable for canonical URL

[benjaoming]

This is probably best solved together with #9423

• Add a new environment variable READTHEDOCS_CANONICAL_URL
• Add some test build that ensures it's correctly defined inside a build environment?
• Update example-mkdocs-basic
• Update documentation
• Mark Inject canonical tag on mkdocs as we do with Sphinx #4820 as fixed through the documentation and example

[stsewd]

This is kind of similar to #8260, but that includes the whole URL