Update docs for 2.1 version of the contracts

[andreivladbrg]

Ref:

• core changelog
• periphery changelog

[PaulRBerg]

@andreivladbrg it would very helpful if you could provide a little more detail about what this task entails beyond just linking to the changelogs.

e.g.

• [ ] Document batch
• [ ] Remove proxy architecture

[PaulRBerg]

Note: once this task is finished, this PR should be merged:

https://github.com/sablier-labs/examples/pull/20