Add upgrade documentation

[swetha1654]

Applicable spec:

Overview

• Added upgrade documentation
• Removed src-docs references
• Updated index.md

Rationale

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated
• The PR is tagged with appropriate label (urgent, trivial, complex)
• The CHANGELOG.md is updated with user-relevant changes.

[erinecon]

Thanks so much for adding upgrade documentation (and for adding the missing Contents section to the index page)!