Add build instructions

See merge request pleroma/docs!4

I Changed a bit in the README and added a short explanation on how to build the docs using this project and what dependencies are needed.
I also added an example of how to build and locally run the docs on a Debian based distro optionally using a Pythin virtual env.

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Add contrast to code snippets

See merge request !2

Add `.tmp` to `.gitignore`

See merge request pleroma/docs!1