improve documentation

[alifeee]

documentation (the readme and https://docs.gspread.org/en/v6.0.0/) has not been looked at in a while. I'd like to look over it, and see if there are any improvements that could be made

[alifeee]

https://developer.mozilla.org/en-US/blog/technical-writing/

[lavigne958]

This is the last issue in this milestone, I feel like it needs some target, we could improve it for sure, first we should have a look to what needs to be done in my opinion.

What do you think if we remove this issue for now from 6.1.0 and move it to 6.2.0 ?

This will allow to release 6.1.0 once #1446 is merged too 🤔

[jrdek]

This is a quick fix, but the documentation for BackOffHTTPClient mentions that it should be passed using the client_factory parameter; looks like that should now be http_client, at least for the oauth method.