Make permissions requirements clear for gunicorn user

[mathemancer]

Problem

As seen in #3002 , a user can correctly follow our docs to build from scratch, but end up with an unusable system since there is not step specifying that the gunicorn user needs to be able to write to some subdirectories under the root directory of their Mathesar system. In particular:

• Gunicorn needs to write to the .media directory to create files when accepting uploaded TSVs etc.
• Gunicorn needs to read from the static directory (I think; someone should double-check my theory on that).

Proposed solution

Add a step or two in the documentation for building from scratch to either chown or chmod the directories appropriately.

Additional context

#3002

[deka014]

Hey, I have installed Mathesar locally from scratch using the docs and reproduced the same bug. Apparently, to import files user gunicorn needs permission to upload/write the csv to ~/mathesar/.media/anonymous$ folder.

I have two solutions :

1. Chmod : Giving write permission to every other users to .media/ folders without changing current ownership
   Using - chmod 777 -R .media/ which results in drwxrwxrwx 3 deep deep 4096 Jul 11 18:52 .media
2. Chown : Changing ownership of the .media folders to gunicorn, Using - chown -R gunicorn:gunicorn ./media which results in drwxrwxr-x 3 gunicorn gunicorn 4096 Jul 11 18:52 .media

Please let me know which one i should proceed with, i will make the changes accordingly. Thanks 👨‍💻

[kgodey]

@deka014 I'd recommend using the solution @mathemancer suggested in #3002 (comment)