Updated install guide

[Zentro]

This updates the install guide to better cover different approaches to installing the game both on Windows and Linux.

[CuriousMike56]

• When installing under Linux I've often had to give execute permissions to the RoR executable along with the RunRoR script, this step should be included in the instructions.
• Could you include steps for installing the development build (zip file and itch.io)?
• There's an absolute link warning:

INFO    -  Doc file 'gameplay/installing-the-game.md' contains an absolute link '/images/win-install-ror.gif', it was
           left as is. Did you mean '../images/win-install-ror.gif'?

Overall it's much cleaner now, I like the use of a GIF for the Windows installer.

[ohlidalp]

Looks really good.

[Zentro]

Applied the requested changes and fixes.