Documentation: add DDBBs documentation section for ReadThedocs.io
related to https://earth.bsc.es/gitlab/digital-twins/de_340/project_management/-/issues/216
The idea is to provide a brief description (summary) of the Autosubmit databases based on the work done for the documentation of the Autosubmit/Autosubmit GUI/Autosubmit API DDBBs, It should be short and focused on explaining the kinds of databases (distributed and central), location (the experiment directory, the as_metadata and the directory for the central DDBBs, explained if the location is parametrized in the configuration), general purpose for each one of the three, and who writes those
Original documentation: https://docs.google.com/document/d/10KoXpY3fy1-zCCOIMIYmmbWhnTWj-3BZ6szgU_QfnmA/edit#