Enhanced note under docs section that libs/tools have their own docs.

Related to issue #1, wanted to make it more explicit that libraries
and other tools have their own docs and stuff under Documentation
section is generic.

Old note:

    Notice that individual projects in the ecosystem have their own
    documentation.

New note:

    Notice that individual test libraries and tools in the ecosystem
    have their own documentation that is typically accessed via their
    project pages.

'Test libraries' and 'tools' in the new note are links to relevant
sections.