Update references to shielded transactions

[ptagl]

After the development of the removal of shielded transactions #628, docs on the repository have been updated to reflect the current status of Zend and remove references to any feature that is not available anymore.

Some sections have been kept and marked as "deprecated" since they may still be useful somehow after the hard fork activation.

[cronicc]

Approved, comments non-blocking

[JackPiri]

The PR is fine to me.
Just mentioning in payment-api.md there are multiple references to privacy features (z_* RPCs, zaddr, zcash-cli, etc) probably we want to get rid of.
Thanks

[a-petrini]

Good for me, I left a minor non/blocking comment

[ptagl]

I've added a couple of commits to address the requested changes, let me know if they look ok.

payment-api.md could be extended to add more information (for instance how the z_mergetoaddress command works), but if it's not crucial, we may postpone this task to another PR.