fix(build): raise clear error if mkdocs.yml config file is missing#12378

RabbitAlbatross wants to merge 3 commits intoreadthedocs:mainreadthedocs/readthedocs.org:mainfrom

RabbitAlbatross:fix-mkdocs-error-messageRabbitAlbatross/readthedocs.org:fix-mkdocs-error-messageCopy head branch name to clipboard