Set up reference documentation with Sphinx

[lengau]

What needs to get done

Create an RTD project and set up sphinx.

• Home page
• Autogenerated reference
  • command help, one command per page
  • Project Model references

Why it needs to get done

So we can link to auto-generated documentation from docs.charmhub.io

cc @tmihoc

[syncronize-issues-to-jira[bot]]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-2981.

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