Add `markdownlint` linter and fix issues. Config is based on the one
from electron's repo with a few rules relaxed.

Adds an upgrade script that automates upgrading installations on Linux from binary releases, so people don't need to reinvent the wheel. Hopefully this leads to less questions about how to upgrade, and consequently less Gitea instances running unmaintained versions in the wild.

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>

* doc: Upgrade from an old Gitea

* update backup steps

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: delvh <dev.lh@web.de>

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: delvh <dev.lh@web.de>

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: delvh <dev.lh@web.de>

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: delvh <dev.lh@web.de>

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: delvh <dev.lh@web.de>

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: delvh <dev.lh@web.de>

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: delvh <dev.lh@web.de>

* update backup tips

* update table header

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: silverwind <me@silverwind.io>

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: silverwind <me@silverwind.io>

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: techknowlogick <matti@mdranta.net>

* Update docs/content/doc/upgrade/from-gitea.en-us.md

Co-authored-by: techknowlogick <matti@mdranta.net>

* Update from-gitea.en-us.md

* Update from-gitea.en-us.md

* Update from-gitea.en-us.md

* Update from-gitea.en-us.md

* Update from-gitea.en-us.md

* Update from-gitea.en-us.md

* Update from-gitea.en-us.md

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>