Adapt the getting started guide to TF-PSA-Crypto 1.0 and Mbed TLS 4.0

[gilles-peskine-arm]

Adapt https://github.com/Mbed-TLS/mbedtls-docs/blob/main/getting_started/downloading.md and https://github.com/Mbed-TLS/mbedtls-docs/blob/main/getting_started/building.md to TF-PSA-Crypto 1.0 and Mbed TLS 4.0.

Note that users can get started with an LTS version, so we should keep instructions for 3.6 around. They can be in the same document or in separate documents. Given the current structure, I think it would make sense to keep a unified downloading.md and to make a new, separate building.md. But that's up for discussion.