Add documentation for common reading (browsing?) workflows/strategies

[neiljp]

ZT has a different feature set to other clients right now, and to some extent the UI is also different.

This could encompass https://zulip.com/help/reading-strategies, but also cover

• use of explore mode / marking-read handling
• looking at topics individually using t
• using z/a for contextual browsing (if possible)
• specific use cases for reading through unreads (eg. n & p)

This issue was prompted by #zulip-terminal>Reading Workflow on chat.zulip.org.