Docs: Add documentation on the verdi storage command

[ramirezfranciscof]

After the structural modifications to move to verdi storage (#5228) and the addition of the maintain subcommand (#4965), the documentation needs to be updated.

[agoscinski]

I checked the doc and could not find any entry about mentioning introducing verdi storage or maintain that was added in the mean time. Probably the storage section in Topics https://aiida.readthedocs.io/projects/aiida-core/en/latest/topics/storage.html is a good section to add this. Also https://aiida.readthedocs.io/projects/aiida-core/en/latest/topics/performance.html could be good to mention verdi storage maintain.

There is already a section about backing up
https://aiida.readthedocs.io/projects/aiida-core/en/latest/howto/installation.html#backing-up-your-data

Some discourse threads that could contain relevant resources for the doc
https://aiida.discourse.group/t/performance-issues-with-verdi-commands-verdi-process-and-verdi-storage/398
https://aiida.discourse.group/t/clean-aiida-storage-after-deleting-the-nodes/155/3