Update crate READMEs

[newpavlov]

• Update MSRV badges.
• Remove the text-based "documentation" link. It's redundant, especially when readme is included in crate docs.
• Remove the "Minimum Supported Rust Version" section. After update to the 2024 edition we have a more relaxed MSRV policy, so the MSRV badge should be sufficient.
• Remove the "SemVer Policy" section. After the MSRV policy change we have the default SemVer policy (i.e. all features and public APIs are subject to SemVer), so we do not need to say it explicitly.