Clean up KNOWN-ISSUES.
Clean up KNOWN-ISSUES.

1) Structure the document.

2) Delete the Github file, and only keep the version on the devsite (https://developers.google.com/tink/known-issues). Having two versions means they will diverge -- and in fact, the AES-CTR-HMAC-AEAD issue in C++ was only listed on github.

3) Remove the point that ECDSA is malleable. This is not an issue, in fact the standard defines ECDSA as malleable. We instead add this to the "Choosing a key type" section.

4) Add fork safety.

PiperOrigin-RevId: 467638967

google/tink,google/tink,google/tink,google/tink,google/tink,google/tink,google/tink,google/tink