docs(agents|workflows): Update litmusctl usage docs

[PrayagS]

What this PR does / why we need it:

This PR adds the following changes in litmusctl documentation,

• Refactor create agent to connect agent.
• Add REGISTRATION field in get agents output.
• Add disconnect agent command under additional commands.
• Add instructions for create workflow command.
• Add other workflow commands viz. get, delete, and describe under additional commands.

Checklist:

• [ ] Fixes #
• [x] Signed the commit for DCO to be passed
• [ ] Labelled this PR & related issue with documentation tag

Signed-off-by: PrayagS prayagsavsani@gmail.com

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