Docs: add info on export format to internals section

[ramirezfranciscof]

Fix #4037
Partially addresses #4450

[giovannipizzi]

Hi Francisco, can you also remove the old content from the previous source (I think it was in import_export/main.rst), either only the content you moved, or all the file, so at the end we know what we didn't migrate yet?

[ramirezfranciscof]

Ok; my original idea was that I could anyways wait until all associated new content was merged and then delete all files in a given "remove" issue at once (seemed better than have each one decide what part to remove of the files), but from the message in the slack and this comment I assume you prefer that each deletes the part that each copies.

PD: It seems that we are having a BIG bottleneck in the CI tests. Maybe we should discuss if there is something that can be done about this in the afternoon meeting...

[ramirezfranciscof]

I updated all references to "export" into "AiiDA archive". There are some inconsitencies due to the references in the actual code (export_version inside the metadata.json, or the command being verdi export migrate) but I don't think there is much I can do about those.

Now should be ready to review @giovannipizzi @sphuber

[giovannipizzi]

Some minor suggestions.

Also, please add a redirect from the old URL describing the data storage to this new URL.

[ramirezfranciscof]

@giovannipizzi for now the redirection works for the full page (so import_export/main.html --> internals/data_storage.html), I have just asked if it was possible to do section redirection (import_export/main.html#export-file-format --> internals/data_storage.html#aiida-archive-format) which is the link that you originally wanted to fix.

[ramirezfranciscof]

For which the answer is that apparently this can't be done, so I guess this is the best we can do.

[giovannipizzi]

Thanks!