fix links to deprecated bitbucket repos

[jschnasse]

This PR fixes some deprecated links I found (and stumbled upon) at https://docs.tech.cessda.eu/ , like

• https://bitbucket.org/cessda/cessda.guidelines.public/src/master/CONTRIBUTING.md --> https://github.com/cessda/cessda.guidelines.public/blob/main/CONTRIBUTING.md
• https://bitbucket.org/cessda/cessda.guidelines.public --> https://github.com/cessda/cessda.guidelines.public
• https://bitbucket.org/cessda/workspace/projects/ --> https://github.com/cessda
• https://bitbucket.org/cessda/cessda.cdc.osmh-indexer.cmm/src/ --> https://github.com/cessda/cessda.cdc.osmh-indexer.cmm