Minor updates to documentation.

[abhishekrb19]

This small change makes updates to Druid's awesome documentation. Added code blocks in some places, change recommended segment unit mb -> MB, weird number rendering by markdown format as can be seen here: https://druid.apache.org/docs/latest/design/segments.html

[vogievetsky]

Looks good to me, will merge after CI

[abhishekrb19]

@vogievetsky, the CI build integration tests are failing with following error:

[INFO] --- exec-maven-plugin:1.6.0:exec (build-and-start-druid-cluster) @ druid-integration-tests ---
Error response from daemon: No such container: druid-historical
Error: No such container: druid-historical
Error response from daemon: No such container: druid-coordinator
Error: No such container: druid-coordinator

I think the other CI tasks are stuck (been at "In progress" for a long time now). These failures should be unrelated to these doc changes, but I am not sure if I can kick off another job.

Also, looked at this PR #8664, and I see similar failures, wondering if the CI/CD pipeline is down?