Add migrate command to docs

[jolheiser]

As title, I've seen this question several times lately.

Feel free to suggest amendments, there's not much to explain about the sub-command. It has no extra options.

[jolheiser]

I also added the gitea convert command to the docs. Same as migrate, there's not much to explain about it other than re-iterating the help-text.

[codecov-io]

Codecov Report

Merging #8842 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8842      +/-   ##
==========================================
- Coverage   41.22%   41.22%   -0.01%     
==========================================
  Files         543      543              
  Lines       69785    69785              
==========================================
- Hits        28772    28768       -4     
- Misses      37324    37325       +1     
- Partials     3689     3692       +3

Impacted Files	Coverage Δ
models/unit.go	62.16% <0%> (-5.41%)	⬇️
modules/migrations/migrate.go	21.22% <0%> (-1.68%)	⬇️
modules/migrations/gitea.go	10.16% <0%> (-1.49%)	⬇️
routers/repo/view.go	37.71% <0%> (-0.88%)	⬇️
models/repo.go	48.57% <0%> (-0.06%)	⬇️
models/repo_list.go	74.27% <0%> (+0.97%)	⬆️
modules/log/event.go	65.64% <0%> (+1.02%)	⬆️
models/error.go	32.98% <0%> (+1.23%)	⬆️
modules/task/migrate.go	28.2% <0%> (+3.84%)	⬆️

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