docs: Remove current docs and link to the new location

All current documentation has been transferred to a new git called
optee_docs [1]. The output from optee_docs will be rendered using Sphinx
and hosted at optee.readthedocs.io [2]. The new documentation git will
also be part of the regular OP-TEE releases. For completeness, it will
also be included in our manifests making up a full OP-TEE developer
setup.

[1] https://github.com/OP-TEE/optee_docs
[2] https://optee.readthedocs.io

Signed-off-by: Joakim Bech <[email protected]>
Acked-by: Jerome Forissier <[email protected]>