Add the github repo url to the doc config

[joshrotenberg]

This adds the lettuce GitHub repo link/information to the top right corner of the docs, handy for getting back to the repo from any documentation page.

Make sure that:

• You have read the contribution guidelines.
• You have created a feature request first to discuss your contribution intent. Please reference the feature request ticket number in the pull request.
• You applied code formatting rules using the mvn formatter:format target. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.

[tishun]

Nice! Thanks!