Add links (badges) to docs.rs and crates.io to readmes

[masklinn]

ua-parser has a link to docs.rs on the home page but that's it, finding the docs to regex-filtered from the repo requires looking it up on docs.rs (or via crates.io).

Also might make sense to have a mini readme at the root linking to the regex-filtered and ua-parser directories / repos / readmes instead of symlinking the ua-parser readme? Just because there's not a single user of regex-filtered out there doesn't mean it needs to be hidden that hard.

https://shields.io seems to have support for crates.io (multiple axis), docs.rs, and gh actions.