Add FAQ to docs

First Q&As:

Q: I am getting the following error: pymongo.errors.DocumentTooLarge: 'findAndModify' command document too large. What can I do?

A: You can change the default setup for background I/O, see https://vre-language.readthedocs.io/en/latest/io.html#background-i-o-operations

Q: I cannot find a model in the database, what can I do?

A: You can use tagging and search the database for known tags, metadata and data: https://vre-language.readthedocs.io/en/latest/query.html

Q: The statements in the model are evaluate in an order that is non-intuitive. I wish certain statements to be evaluated now and other statements only later.

A: You can consider using the --on-demand option.

Q: Slurm does not accept jobs with --account but texts adds my username as the default account.

A: Open an ipython session and run the following code:

from virtmat.middleware.resconfig import get_default_resconfig, get_resconfig_loc
cfg = get_default_resconfig()
cfg.default_worker.accounts.append(None)
cfg.default_worker.default_account = None
cfg.to_file(get_resconfig_loc())

Add HELP in the top-level index, add the FAQs there and move the support contact also there (like https://vre-middleware.readthedocs.io)