Ensure that github links in the docs point to the release branches rather than master. Prevents links becoming stale.

Two approaches:

1. Regenerate the documentation via hugo. For all versions: 3.1 - 3.9
At some point the versions of hugo changed from v0.13 to v0.25 - so both would be required to rebuild the documentation.

• Check out the maintenance branch eg: 3.1.x
• In docs/reference/config.toml update the source code version to point to the maintenance branch.
• In docs/reference/data/mongodb.toml update the githubBranch to point to the maintenance branch.
• Commit the changes and push to the maintenance branch
• Regenerate the code and copy over the code in the gh-pages/3.1 directory.

2. Just change each release docs in the gh-pages:
Grep for the incorrect links and update them eg:

• grep -rl 'mongo-java-driver/tree/master' ./3.1 | xargs sed -i 's/mongo-java-driver\/tree\/master/mongo-java-driver\/tree\/3.1.x/g'

*NOTE* From: 3.7.x mongo-java-driver/tree/3.7.x/driver becomes: mongo-java-driver/tree/3.7.x/driver-sync

Once that is done, the QuickTours (for sync and async) link needs to be fixed in each maintenance branch to point to the correct docs.
eg: https://github.com/mongodb/mongo-java-driver/blob/3.1.x/driver/src/examples/tour/QuickTour.java#L57 and the async version.