Make unit test helpers file into a public submodule

[tskisner]

• Delete old tutorial directory
• Move mkdocs to final docs location
• Add github workflow for deploying docs
• Convert _helpers.py into a submodule. Small tweaks to create_outdir helper.
• Change imports across all unit tests
• Run format_source.sh

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[tskisner]

This PR has many trivial changes, mostly moving and deleting files.

[keskitalo]

We have a lot of unit tests!

The fact that we are importing methods from the interactive module in every unit test has me wondering if the module name is descriptive or not. Obviously not an issue for this PR.

[tskisner]

Summarizing offline discussion:

• The toast.interactive module should contain just things intended to run from a notebook (i.e. that require ipython)
• The rest of the "helper" functions that setup synthetic test cases are primarily useful for the unit tests, even if we also want to use them for documentation tutorials. For this code, it is fine to just make a public toast.tests.helpers module that replaces the old, large _helpers.py. In principle this could also grow to include more "cook book" style involved examples- since we want to test that code as well.
• I will do this rearranging and rebase / squash to reduce the noise in this PR.