docs: add how-to docs

[cbartz]

Overview

Add "How to upgrade" documentation.

Rationale

To inform the user of the usual way to upgrade.

Checklist

• The charm style guide was applied.
• The contributing guide was applied.
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex).
• The changelog is updated with changes that affects the users of the charm.
• The application version number is updated in github-runner-manager/pyproject.toml.

[erinecon]

Thanks so much for adding upgrade documentation!