Android "Obtainium" installation method

It'd be nice if one of the installation methods supported were [Obtainium](https://obtainium.imranr.dev/), it allows automatic downloading, installing and updating of Android applications directly from their source, i.e GitHub, and can have configurations encoded into regular HTTPs URLs.

There's also [a badge](https://github.com/ImranR98/Obtainium/blob/main/assets/graphics/badge_obtainium.png) that could be used to represent this installation method.

It's a regular URL so it can copy the code / extend from installation methods like windows_installer which also just use a HTTP url.

Start by locating the existing windows_installer installation method in the matrix.org website and compare how its regular HTTPS URL is represented. Add an Obtainium installation option using the linked badge as appropriate; it is done when Android visitors can find and use the Obtainium URL alongside the existing methods.

Written by the indexing model from the issue text.