Update Readthedocs with information on dnf/dnf5 docs

[inknos]

The PR does not affect readthedocs directly. we are now changing what dnf means, though, before dnf was referring to dnf<5 now it's dnf5.

I was just saying, we have readthedocs which is generated by the same sources. so, should we keep dnf->dnf4 or switch to dnf->dnf5?

it is tricky since the online docs are probably referenced somewhere.

I'd keep dnf.readthedocs.io for dnf4. If it's feasible and straightforward, we could consider adding a mention and link to the new dnf5 pages, e.g. on the title page. What do you think?

Originally posted by @jan-kolarik in rpm-software-management/dnf#2072 (comment)

• update docs referencing to both web pages
• update man references, if any, in the web docs with the appropriate command (e.g. man dnf4 and man dnf)