In directive documentation, version_added supports only numbers at the moment. That is caused by the fact that docs/generate_directive_docs.py uses float number comparison and doesn't display "Added in version X" statements if the directive is too old (older than TO_OLD_TO_BE_NOTABLE).

However:
* we need to use strings in versions, like 0.7git234ceg4
* float comparison is broken anyway, 3.9 < 3.10
* TO_OLD_TO_BE_NOTABLE is not used anyway at the moment

Let's implement the following simple approach:
* allow strings in version_added
* remove TO_OLD_TO_BE_NOTABLE and the version comparison code
* if we think some directive is "too old to be notable", we can simply remove version_added field from that directive
* therefore, verify that version_added field is not mandatory in directive documentation and the document renders fine without it