Docs: New App plugin tutorial

[academo]

What this PR does / why we need it:

Introduces the app plugin tutorial with updated code and content inspired from https://github.com/grafana/tutorials/blob/master/content/tutorials/build-an-app-plugin.md

Which issue(s) this PR fixes:

Closes: https://github.com/grafana/plugin-tools/issues/555

Fixes #

Special notes for your reviewer:

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[josmperez]

Please review my first round of edits in today's commit. I will take a second pass through it soon as well.

[academo]

@josmperez the changes look great. Please go on with your review.

[academo]

Added some quick comments, in terms of additional things we could cover or reference as appropriate:

* app backends and callresource

* healthcheck

* bundling dashboards

* e2e tests (next step)

I think these topics require their own page with more extensive content

I created two issues to work in those docs

https://github.com/grafana/plugin-tools/issues/964 https://github.com/grafana/plugin-tools/issues/963

[academo]

@josmperez please take another look at the app tutorial when you can https://github.com/grafana/plugin-tools/pull/914

no drastic changes since your last edit mostly expanding on some sections.