docs(payjoin): highlight {v1,v2}::{send,receive} as primary API entry

[Mshehu5]

This PR address some issues in https://github.com/payjoin/rust-payjoin/issues/865 mainly this issue :

the top level payjoin module doc should: refer to the {v1,v2}::{send,receive} modules for the actual API, without prior knowledge it's hard to tell that these are the main entry points into the API they're just in the middle of the modules list explain refer to payjoin-cli crate as an example

Pull Request Checklist

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• [x] I have disclosed my use of AI in the body of this PR.
• [x] I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[coveralls]

Pull Request Test Coverage Report for Build 19823725348

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.539%

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Totals
Change from base Build 19686158876:	0.0%
Covered Lines:	8988
Relevant Lines:	10759

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💛 - Coveralls