In #5678, I thought that the extra README that eventually ended up in
`cvat-sdk/docs/README.md` was unused, which turned out to be not quite
true. One section from it _was_ used to generate the online API index
page. Since I removed the file, the online docs now fail to build.
Rather than restore the entire README, restore just that one section as
a standalone file. This achieves the same result as before, but with
simpler code.
Roman Donchenko
Maxim Zhiltsov