add directory for the documentation

This PR migrates the documentation from github to gitlab. The documentation is then rendered at https://data-custodian.gitlab.io/custodian

It was considered to preserve the history of the previous repo on https://github.com/SDSC-ORD/Custodian_documentation, but it seemed then not straight forward enough and also the documentation is anyway refactored, therefore the branch builds it up from scratch:

it uses:

• pdm as python package manager
• the version is set manually and adapted to the Custodian version
• sphinx as documentation tool
• pydata sphinx theme as theme for sphinx

the ci/cd pipelines have been adapted adapted to build the gitlab page the links to the documentation in the Custodian readme have been adapted to point to the new documentation.