docs: Update xmtp_mls README.md to remove warning tags and add missing information

[VolodymyrBg]

This commit improves the xmtp_mls/README.md documentation by:

1. Enhancing the documentation for the xmtp-id crate with detailed information about:

   • Identity management for XMTP inboxes
   • Wallet address verification for both EOA and Smart Contract Wallets
   • Legacy V2 identity migration support
   • Core types and traits
   • Smart contract detection utilities

2. Adding specific information about the TLS configuration used by XMTP:

   • Clarified that XMTP uses the default TLS configuration provided by Rustls
   • Specified that this includes the system's root certificate store
   • Mentioned the modern and secure ciphersuites supported (TLS 1.2/1.3 with AEAD ciphers)

3. Removing warning tags that indicated missing information

These improvements make the documentation more complete and eliminate TODOs that were present in the file.