[GRASS-git] [OSGeo/grass-website] 172582: content: fix broken links; master -> main; add stu...

[Markus Neteler]

  Branch: refs/heads/master
  Home:   https://github.com/OSGeo/grass-website
  Commit: 172582300d9eb05c3c8644e617e266ec0ae7ea0e
      https://github.com/OSGeo/grass-website/commit/172582300d9eb05c3c8644e617e266ec0ae7ea0e
  Author: Markus Neteler <neteler at gmail.com>
  Date:   2022-04-25 (Mon, 25 Apr 2022)

  Changed paths:
    M content/about/credits.md
    M content/about/history/releases.md
    M content/about/mirrors.md
    M content/contribute/_index.en.md
    M content/contribute/development.md
    M content/download/docker.en.md
    M content/learn/overview.md

  Log Message:
  -----------
  content: fix broken links; master -> main; add student grants (#301)

While also this repo should be updated from master -> main in a different PR,
this PR updates links to various GRASS GIS GitHub repos in this regard.

Link update to GRASS GIS addons repo.

Commented out (currently?) broken mirror sites.

Also added: "Student Grants".