feat: add book finished date template variable.

[bandantonio]

Description

This feature also includes:

• Update to database migrations to include the new field.
• Update to documentation to include examples of the new variable.

Fixes #64

Checklist

• I accept the Code of Conduct
• I have performed a self-review of my code for consistency and potential leftover debug logs, commented out code, etc.
• I have added tests that prove that my feature works or that the fix is effective
• I have checked that new and existing unit tests pass locally with my changes
• I have updated the documentation accordingly (if necessary)
• I confirm that this PR DOESN'T change the plugin's version either in the manifest.json or package.json files
• I have checked that my commit messages are descriptive and follow the Conventional Commits standard
• I have enabled the checkbox to allow maintainer edits so the branch can be updated for a merge.