Improve documentation around disk space usage

[kellyfj]

Motivation:

• Figuring out how disk space is consumed by Pulsar is complex

Modifications

• Clarified that retention policies are on a per-topic basis
• Created entry in the FAQ on how to debug disk space issues (will need some help on this)

Verifying this change

Just a documentation change

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): no
• The public API: no
• The schema: no
• The default values of configurations: no
• The wire protocol: no
• The rest endpoints: no
• The admin cli options: no
• Anything that affects deployment: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable
• If a feature is not applicable for documentation, explain why? Just doc change
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[Jennifer88huang-zz]

1. do we add "TBD" in our docs?
2. It's a long time we do not maintain this faq file.