Various updates to the 13.0 readme.

• Remove spurious quotes from attributes.

• Update the descriptions of supported architectures including adding aarch64 and RISC-V and removing sparc64.

• Suggest installing packages via pkg(8) rather than the installer.

• Claim 13.0 is a release and not a developer snapshot.

• Trim stale info about obtaining FreeBSD and instead point the user at the "where/" page on the main website.

• Switch mailing list from current@ to stable@.

• Add an explicit link to bugs.freebsd.org and remove mention of send-pr(1).

• Just assume bsdinstall works on all platforms.

• Other minor edits.

Reviewed by: gjb (earlier version)
Differential Revision: https://reviews.freebsd.org/D29243