Adding installation instructions FFmpeg on Linux and changes of rst file

[Savraska2]

I propose the following division into chapters:

Using FFmpeg with OpenToonz ├── What is FFmpeg? ├── Installing FFmpeg for Windows │ ├── Downloading │ └── Installing ├── Installing FFmpeg for Mac │ ├── Downloading │ └── Installing └── Installing FFmpeg for Linux

instead of the one used now:

Using FFmpeg with OpenToonz ├── What is FFmpeg? ├── Downloading FFmpeg │ ├── Downloading FFmpeg for Windows │ ├── Downloading FFmpeg for Mac │ └── Downloading FFmpeg for Linux └── Installing FFmpeg ├── Installing FFmpeg for Windows ├── Installing FFmpeg for Mac └── Installing FFmpeg for Linux

The chapter is divided into sections on the principle of the operating system, and not "Downloading/Installing". There are several reasons for this:

• Allows you to avoid unnecessary hyperlinks from section "Install" to section "Download" regarding same operating system. It is assumed that the user will select the installation section for the OS he needs and follows the instructions. The file "installing_opentoonz.rst" is arranged in the same way.
• Installation on Linux is done by a single command, bypassing the site. Where to put these instructions is not entirely clear.

[WolfInABowl]

I agree with the proposed changes Thank you very much for the Linux instructions!