docs: Tutorial updates

[erinecon]

Three updates that expand over all four tutorials. These changes were suggested from one or more UX sessions.

Summary of changes:

1. If the user needs to open rockcraft.yaml or charmcraft.yaml to update the architecture to arm64, have them also comment out amd64. While there's no harm or issue with having both architectures available, for novice users, this change will eliminate any possible concern or worry over including both architectures in the YAML file(s).
2. Provide more explanation and reassurance about the web app entering a blocked state when adding the PostgreSQL database.
3. Rephrase the "tear things down" section to tell the users that they can just delete the whole instance if they're not going to keep the VM.

Additional reviewer: @jdkandersson

[medubelko]

@lengau Can Charmcraft run on ARMhf systems?

[lengau]

Technically yes, but the last time snapcraft.io reports a nonzero number of users on armhf (2) was April of last year. One would have to deploy their own juju runners to run charms on armhf AFAIK.

[medubelko]

Note for work in the next cycle: In other tutorials I made this a required step, even though it was optional. We're not to fork in tutorials.

[medubelko]

Suggested change

	If you'd like to manually reset your working environment, you can run the
	following in the rock directory ``~/fastapi-hello-world`` for the tutorial:
	If you'd like to manually reset your working environment, you can run the
	following in the ``~/fastapi-hello-world`` directory:

This flows more cleanly to me, but I might be missing the meaning.