Git Developer Pages cleanup

Hi everyone,

For sometime I have been thinking about cleaning up the Git Developer Pages web site (https://git.github.io/), and making some of the documentation there more generic and possibly more useful for all kind of newcomers.

So I pushed the following commits:

* 17b7b3e SoC: remove 2017 SoC material from the navbar
* 4e01d0a SoC: remove 2018 SoC material from the navbar
* 7c13096 Outreachy: fix titles

Old GSoC and Outreachy material can now only be accessed through the Historical Summer of Code Materials (https://git.github.io/SoC-Historical/) link.

* 9f46f72 Outreachy: rename Outreachy-2016-May to Outreachy-12

An old Outreachy related file now has a similar name as other Outreachy related files.

* 717ec3c Add General-Microproject-Information.md

Extract information from https://git.github.io/SoC-2019-Microprojects/ to make it independent of GSoC or Outreachy. I think it's really better to have a separate document and just link to it, instead of copy pasting it each time.

* 5f6b197 General-Microproject-Information: newbie related improvements

As the document can be useful to newcomers, tell a bit about that in the document.

Thoughts?

Thanks, Christian.