Add GitHub DCO-check workflow and fix TF-M URLs (#115)

* github-ci: Add DCO sign-off enforcement workflow

Introduce a `dco-check` job in
`.github/workflows/ci.yml`
that runs on pull request events. The job loops over
every commit in `origin/main..HEAD` and fails immediately
if any commit lacks a properly formatted

    Signed-off-by: <Name> <email>

footer, ensuring full compliance with our Developer
Certificate of Origin (DCO) policy.

Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>

* docs: Redirect TF-M links to Read the Docs URLs

Trusted Firmware-M has moved its public documentation
from `tf-m-user-guide.trustedfirmware.org/` to
`trustedfirmware-m.readthedocs.io`. Hence, switching to
the new domain.

Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>

* docs: Add the missing development environment requirements

Some of the newly added dependencies are missing in the
development environment requirements document. Hence, adding
these new dependencies.

Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>

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Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>