Update documentation on how to run parsers

• Update documentation on how to run individual parsers. The separate page (howto/programmatic/local_parsers.py) was removed, and the section is now a part of the parser how-to (howto/plugins/parsers.html#running-a-parser).
• Remove hard-coded tables that contain the parsers/schemas/normalizers created by FAIRmat. Replace with a link to the FAIRmat organization GitHub page at the introduction paragraphs of howto/plugins/plugins.html.
• Mention in the introduction paragraph of howto/plugins/plugins.html that the "About" page of each distro lists the activated plugin entry points.