Refine Repo README and payjoin lib.rs

[mehmetefeumit]

Pull Request Checklist

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• I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

Overview

This PR contains a couple of changes related to the higher-level documentation in the repository:

1. Refine the existing repo-level README with up-to-date list of crates, concise information, etc.
2. Refine the lib.rs of the payjoin crate so that rust docs are tidier and more concise.
3. While I am at it, updates some Receiver documentation.
4. Finally, adds a Discord server badge to the repository README.

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Closes #1216

[coveralls]

Pull Request Test Coverage Report for Build 20638228938

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 82.903%

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Totals
Change from base Build 20617301558:	0.0%
Covered Lines:	9669
Relevant Lines:	11663

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💛 - Coveralls

[DanGould]

Epic much needed changes to our docs. Where I made suggestions about specific wording, I tried to offer a suggestion that you can accept by simply clicking. My main concern is the addition of a payjoin/readme including examples which can't be checked against the compiler. I think it's more rusty to leave the top-level readme as the payjoin crate readme, as a sort of entrypoint into the project. Then, once someone opens docs.rs they can see the payjoin module's lib.rs rustdocs that are checked by the compiler for examples and details.

Once those are checked I think this is ready to go.

[DanGould]

It's worth mentioning high-level bip77 specific changes here and definitely in the ohttp-relay readme. TODO

[mehmetefeumit]

@DanGould Thanks for all the feedback. I addressed all of them.

After seeing the rust-bitcoin and rust-lightning references you shared, I decided to keep the lib.rs content after the change minimal, with a very short summary of the crate and the features available. Otherwise, all of the other changes are going to be similar to what you first reviewed, plus the changes you recommended.