add docs to develop dashboard and shell apps within ood itself

[Oglopf]

Modify this link to include the branch name, and possibly the page this PR modifies:

https://osc.github.io/ood-documentation-test/ood-in-ood-dev/

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This PR adds sections on how to use the sandbox capability of OOD to do development work, from within the production system, on a separate dev dashboard or shell app.

This also includes tips in presenting all this in how you can set new environment variables for this dev dashboard along with how to set your very own configuration directory for the dev dashboard as well.

Similarly, the shell app entry shows a user how to make their own dev shell, how to make changes to this shell, and then how to rebuild correctly to see the changes.

I'd like to add some more to this such as work on the Files app as well, the goal being to show the world how we do this internally at OSC.

[Oglopf]

To-do

• add some pictures for the dev dashboard with color variation
• cleanup wording
• a couple more advanced examples around customization.
• the shell app page is are bones. I'd like to enrich it a bit still and walk through a rebuild when changes are made

To be clear, the goal here is that these docs would largely be used for onboarding and community dev work. Now, if a new dev comes on board, we can point them to our docs and get them working rather than have many places with many tasks which we expect them to ingest and compose correctly in their head. But, that's just analysis for anlysis' sake. This gives the moves up front, the algebra to build is right there and hopefully this creates a much faster on ramping.

This also let's user's who've fallen into the usual trap of "I started this on a VM, showed it to a researcher, and next thing knew it was a service!" problem. Now, they can at least have a dev environment to work in rather than posting every question on the Discourse, thereby freeing us up more as well.

[Oglopf]

I think I'm happy with this at this point and good with a review and pulling it in. We can enrich the entries more if desired later, but I think this at least gives good fundamentals on how to do this type of work on ood.