Add k8s container

[maeve-fpf]

This is another PR like freedomofpress/securedrop-docs#295 (please read that for an explanation of these changes).

It would be smaller than freedomofpress/securedrop-workstation-docs#96, except it moves everything applicable under a docs directory for consistency with the other two repos. I ran into a strange circular symlink problem without making that change, and it seemed like a good idea anyway. However, this may need more review than the other two for that reason. I've run make docs and make docs-lint and they seem correct.

This is deployed at: https://staging-support-docs.securedrop.org/

Like both other repos, CI is currently set up to trigger on a test branch of this repo, so nothing will happen immedately when this is merged into main.

[eloquence]

Somehow Sphinx gets stuck in an infinite loop for me when building the container:

Your branch is up to date with 'origin/main'.
Running Sphinx v4.2.0
/roc/1/cwd/general_support.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/index.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/encrypted_email_setup.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/using_redmine.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/creating_your_account.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/troubleshooting.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/encrypted_email_troubleshooting.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/encrypted_email_overview.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/encrypted_email_do_not_want.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/general_support.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/index.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/encrypted_email_setup.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/using_redmine.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/creating_your_account.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/troubleshooting.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/encrypted_email_troubleshooting.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/encrypted_email_overview.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/encrypted_email_do_not_want.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/proc/1/cwd/general_support.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/proc/1/cwd/index.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/proc/1/cwd/encrypted_email_setup.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/proc/1/cwd/using_redmine.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/proc/1/cwd/creating_your_account.rst: WARNING: document not readable. Ignored.
/roc/1/cwd/proc/1/cwd/proc/1/cwd/troubleshooting.rst: WARNING: document not readable. Ignored.

and so on, with ever deeper nesting

[eloquence]

Works as expected when testing with the temp commit on staging-ci-test.