[REQ] Manually traced requirements

[mhatzl]

Not all requirements can be traced.
To still allow some form of tracing, it should be possible to annotate requirements that they require manual verification.

A possible approach would be to add a manual flag to the references list instead of the ref-cnt.
This would also integrate well with the general ref-cnt approach.

Example:

# my_req: Some Title

**References:**

- in branch main: manual

At some point, it might be possible that references are added for manually verified requirements.
In that case, the manual flag must not be removed, because manual verification might still be necessary,
but the ref-cnt should be added after the flag to indicate that references in code exist.

Example:

# my_req: Some Title

**References:**

- in branch main: manual + 1