Redirect to arm-software site for PSA crypto specifications

[gilles-peskine-arm]

We're currently serving several PSA crypto-related specifications at https://armmbed.github.io/mbed-crypto/psa/index.html, from https://github.com/ARMmbed/mbed-crypto/tree/master . There is now an official PSA specification site at https://arm-software.github.io/psa-api/crypto/ . Going forward, we should refer people to this site.

The goal of this issue is to decide what we do for the various things hosted at https://armmbed.github.io/, which we'd like to eventually retire.

• For new versions of official PSA specifications, we should refer to the new site directly.
• Our copy of the existing versions of the specification, and the direct link to the latest version, have high Google search rank. We should set up some kind of redirect. Unfortunately GitHub Pages don't support 301 redirects.
• We need to update our own links at some point (README.md, getting-started.md, several draft specs…)
• What about draft driver specifications?
• The new site isn't going to host an HTML copy of the 1.0 API spec, what do we do about this?
• The landing page at https://armmbed.github.io/mbed-crypto/psa/index.html should be hosted at mbedtls-docs and served with our other stuff at readthedocs.