doc: update requirements

breathe: for simplicity, require versions > 4.30 (lower versions have
known issues, so do not take risks).
Sphinx: start requiring versions >=4.x. Keep with compatible versions,
since Sphinx major updrages can easily break extensions, themes, etc.
sphinx_rtd_theme: upgrade to >=1.x. Again, keep with compatible versions
since we have style customizations that can likely break on major
upgrades.
pygments: Allow any version >=2.9 (version that introduced DT support).
We do not have strong compatibility requirements here.
sphinx-notfound-page: Remove any requirements, we do not have strong
requirements for this one.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>