[docs] Add documentation for dev-forget-channel

[bitkarrot]

Hi,

First attempt at the docs for the above command. I did run make doc-all and the markdown was generated correctly, but not sure about the *.requests.json.

@ShahanaFarooqui

Issue reference:
#6923

[bitkarrot]

@ShahanaFarooqui I am failing to rebase this and I have run out of battery ( power outage here for next 24 hrs).

Apologies if i messed up the branch in my attempts to rebase and fix the merges

[ShahanaFarooqui]

@bitkarrot, Thank you for the PR, I will re-base and review this PR once the PR #6995 is merged.

[ShahanaFarooqui]

@bitkarrot I re-based the PR on master and cleaned up multiple #empty doc/makefile update commits.

There were some more updates required after PR #6995 merge:

• Merged request and schema jsons into single lightning-dev-forget-channel.json
• Deleted lightning-dev-forget-channel.7.md file because it is fully generated by the script now.

I did change below details in the json schema too:

• Edited required array according to the command
• Types and description of request and response params
• Added errors list returned by the command

Please check and let me know if you need more explanation.

[bitkarrot]

Thanks @ShahanaFarooqui !