Convert All Documentation Links to Relative Path Links

[biergaizi]

This PR converts all documentation links to relative path links. This way they also work in local documentation. A few broken links were also found and fixed.

P.S: I also noticed that there is a direct reference to https://scikit-rf.readthedocs.io/ in contributing/index.rst as the project's documentation site, which was not mindlessly converted, it's preserved as-is as it should.

[jhillairet]

nice and useful fixes!

[Vinc0110]

Cool! Did you build the docs locally and checked if there are any warnings related to these changes? (There are all sorts of warnings, unfortunately, but maybe you can have a quick glance.)

[biergaizi]

I did. After the documents are changed, I tried building the docs repeatedly while watching for any warning from the console. I also tried clicking all the links in my local browser. It took me quite some minutes to figure out the correct syntax for all the links.

Now the only warning seems to be (this is from the CI but it's consistent with what I've seen):

/home/docs/checkouts/readthedocs.org/user_builds/scikit-rf/checkouts/713/doc/source/tutorials/NetworkSet.ipynb:9: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/docs/checkouts/readthedocs.org/user_builds/scikit-rf/checkouts/713/doc/source/tutorials/NetworkSet.ipynb:10: WARNING: Line block ends without a blank line.

I don't understand what's going on. But I believe this warning already existed before the link change and it's unrelated. I don't notice other warnings otherwise. Still, it's all manual changes and manual testing. So it's possible that I've missed something.