Flask docs

[jdkandersson]

• [x] Have you signed the CLA?

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Fixes https://github.com/canonical/rockcraft/issues/587

Improvements to the documentation for Flask support

[jdkandersson]

• Add more information to the reference docs about the extension
• Move most of the content out of how to except for prime and deb into the tutorial

[jdkandersson]

For the rock size, talk about the rock on disk and include a note that says the size on docker is the uncompressed size

[jdkandersson]

Change the prerequisites to instruct people to install, fresh Ubuntu 22.04, can use multipass, make sure snap is available and working, make sure lxd is installed and inited, docker snap

[jdkandersson]

Add an explanation of project details and what we are trying to accomplish - nice to have

[tigarmo]

Thanks a lot everyone! I'm going to merge this as soon as it passes