Improve presentation of available package parsers docs

[pombredanne]

In https://scancode-toolkit.readthedocs.io/en/stable/reference/available_package_parsers.html the table is too wide and forces to scroll.
I think we need a different presentation entirely.
One idea:

• a top level table with one row each supported package type:
  • package type, type description, link to a secondary section below
• then for each type, a list of repeated small tables, one for each parser/file type:

Package type	pypi
Description	lorem ipsum
Path Patterns	lorem ipsum
Package type	lorem ipsum
Datasource ID	lorem ipsum
Primary Language	lorem ipsum
Documentation URL	lorem ipsum