During the AK website upgrade, changes made to kafka-site weren't migrated back to kafka-docs.
This PR is an initial attempt at porting the changes to kafka/docs, but it does not include the streams changes. Those will come in a separate PR.
For the most part, the bulk of the changes in the PR are cosmetic. Only the introduction.html has substantial changes, but it's a direct port from the live documentation.
For testing:
I reviewed the PR diffs
Rendered the changes locally
Reviewers: Matthias J. Sax <mjsax@apache.org>