Remove explicit target links because they can't be translated

[ccordoba12]

We need to remove targets like contributing guide in the line below because they are in the middle of a sentence, so they need to be translated:

spyder-docs/doc/index.rst

Line 109 in 0b3885d

You can follow our `contributing guide`_ to set up a development environment, and you can get involved with the project through our `Github repository`_.

I guess we need to replace them with inline links instead.