[Doc]: Be more specific on dependencies that need to be installed for a "reasonable" dev environment

[timhoffm]

Documentation Link

https://matplotlib.org/devdocs/devel/development_setup.html#install-dependencies

Problem

Most Python dependencies will be installed when setting up the environment but non-Python dependencies like C++ compilers, LaTeX, and other system applications must be installed separately.

Suggested improvement

This is not actionable.

"most" and "non-Python dependencies like [...] and other system applications" makes impossible for a user to know what to install. Digging through following links is cumbersome.

We may not need to give detailed description, but should mention what to additionally install manually (or ensure it's there) for a reasonable working installation, e.g. (not checked for completeness):

You additionally need

• for a minimal working development environment: a C++ compiler
• for building the docs: Graphviz and a LateX.

The full list of required and optional dependencies is available here:

[current links]

[story645]

Your suggestion sounds good to me? Change it to something like:

Python dependencies were installed while setting up the environment. 

Additionally, the following must be installed: 

* Build dependencies (as a link out)
* Documentation build dependencies (as a link out)

I don't want to get overly specific here (listing the specific libraries) just to keep the dependencies doc as the source of truth for this. Alternative is using includes to copy whole chunks, but I think we definitely don't want to do that.

But also I think the bigger issue is that the non python stuff should be better separated/flagged on the dependencies page.

ETA: which actually, if we better seperate the Python from the none python, then the includes are actually pretty small/becomes reasonable.