Finalize Documentation

[Radonirinaunimi]

This PR aims to address once and for all the issues regarding missing/unclear documentations summarized in #393.

Remaining Todo:

• fix upstream Python documentation that does not show some modules
• document the various codes that use PineAPPL

[Radonirinaunimi]

The final piece missing in this PR which concerns the list of programs that use PineAPPL is added in b35684a. I used an itemized list before and didn't look good (at least to my taste) and hence why I went for the table. Please have a look.

[cschwan]

I think I also like the table better - I tried a list myself.

1. Should we really mention APPLgrid and fastNLO? Technically speaking we're using them, not the other way around.
2. APFEL++ and EKO should get links to their homepages.
3. I'm not sure I like the intermediate headings. If you get rid of them,
   • try to use markdown tables
   • sort the entries alphabetically and
   • merge the PineAPFEL entry.

[Radonirinaunimi]

I believe I addressed all your comments. Furthermore, the HTML table might have been indeed too much, so I removed it and replaced with a standard markdown.

If you are happy with this, I believe this PR could finally be merged.

[felixhekhorn]

Furthermore, the HTML table might have been indeed too much, so I removed it and replaced with a standard markdown.

I would have put the table below citation because a) citation instructions are clearly more important and b) the list is expected to grow, so it is better at the end

[cschwan]

I agree!