fix:Fixed broken link and restructuredtext warnings

[ziafazal]

This PR has changes to fix broken link to Supports Events in documentation and fixes warning which break this command

 python setup.py check --restructuredtext --strict

resulting in failure to generate documentation.

[feanil]

@ziafazal this fixes some of the issues, but if you look at the output of make html in /docs you'll see that there are a bunch more similar issues.

eg. A lot of the links on the Supported Events page also have the same broken link problem. I think the links need to be updated to the :doc: type links for referencing other RST files in the same book throught this project.

Totally fine to merge this as is, but it hasn't fully solved the linking issues in the docs.

[feanil]

Suggested change

	List of supported edx events can be found in `Supported_events <https://github.com/openedx/event-routing-backends/blob/master/docs/event-mapping/Supported_events.rst>`_ along with their mapping onto xAPI and Caliper format.
	List of supported edx events can be found in :doc:`Supported Events <event-mapping/Supported_events>` along with their mapping onto xAPI and Caliper format.

We shouldn't link off site for documents that are in this repo, we should be able reference them directly like this.

[ziafazal]

we explicitly link off site here because Supported events page on github links to a lot of other json documents which are on github. If we don't link off site here we'll have to change all those links pointing to github in the Supported events page.

[feanil]

Hmm, I think the correct thing to do would be to correct all the github RST links to be relative links to stay in the sphinx docs as much as possible except for the actual samples. Having some rendered RST and some raw RST seems like a weird mix.

[ziafazal]

Yeah all RST links are relative link except this link in getting_started.rst and reason for that is it needs to point to github.com version of Supported_events.rst instead of sphinx version. Most of the links in Supported_events.rst are relative and point to samples which reside on github.com too. If we make this link relative here and point to sphinx version of Supported_events.rst then we'll have to make all samples links absolute since they are on github.com so I think keep a single link absolute(offsite) here is better than option. One more thing raw RST of Supported_events.rst looks better than rendered RST.

[feanil]

Note, I mean fine to merge after you accept my suggested change which will fix this link more correctly.