fix(docs): add support for readthedocs

- Remove cmake variables from conf.py.in and rename it to confy.py
- Remove dependency on package plantuml
- Move tools/requirements.txt to docs
- Scale down image cca_software_arch.png

These changes make documentation build compatible when built using
cmake or readthedocs.

Signed-off-by: Arunachalam Ganapathy <arunachalam.ganapathy@arm.com>
Change-Id: I4331e6a435b25d1c50439ac85e396737d8042feb
7 files changed