Fix obsolete content from documentation

Remove mention to obsolete gen-list-without-flagging Ant target.
Normalize relative paths in mapref #1683
Clean maplink stylesheet
Clean maplink XSLT
Use tunnel parameters in maplink XSLT
Use () instead of #none# in maplink XSLT
Fix sibling links in map link #1683
Update library dependencies #1791
Upgrade to 2.0 release
Fix issues reported by IntelliJ IDEA
Remove NP plug-in #1512

Remove NP plug-in code, as most of the plug-in functionality has been
migrated to default preprocessing plug-in.
Merge version 2.0 release notes updates from pull request
Create LICENSE.md
Fix compatibility with Saxon-HE 9.6
Merge ReadMe updates for v2 #1792
Merge Release Notes updates for v2.0 #1749
Fix documentation issues
Fix bug in HTML table generation

Introduced in 73fdc39d77e41e4f358534c882607b16672fc319
Minor edits to 2.0 release notes
Update subject scheme map w/ revised version from @keberlein