Update BitBucket Importer docs
As mentioned in gitlab-com/www-gitlab-com!130982 (comment 1631607547), with the most recent changes our BitBucket importer docs seem a bit outdated.
Let's review importer documentation for BitBucket Server and BitBucket Cloud and add missing details, such as items that can be imported (including the milestone the feature was introduced). Also, there may be an opportunity to clean up or docs. For example https://docs.gitlab.com/ee/user/project/import/bitbucket_server.html#import-your-bitbucket-repositories could be merged into the "From Bitbucket Server, you can import" part.