RFR: 8373909: JSpec and ToolGuide taglets use incorrect relative path

[Dan Smith]

Fixing the JSpec and ToolGuide taglets so that they produce correct links, no matter what file the tag appears in.

This is a workaround solution, sharing the necessary info via a ThreadLocal public static field in an internal API. See [JDK-8373922](https://bugs.openjdk.org/browse/JDK-8373922) for an API enhancement request that would make this possible without relying on internal API.

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Commit messages:
 - whitespace fixes
 - restore test/docs/ProblemList.txt (to be fixed with different issue)
 - Simplified command line
 - clarifying exceptions/comment
 - Merge branch 'main' into 8373909
 - use same convention in ToolGuide, tidying up
 - Share current path with JSpec to get accurate links to root

Changes: https://git.openjdk.org/jdk/pull/28878/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28878&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373909
  Stats: 84 lines in 4 files changed: 18 ins; 54 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/28878.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28878/head:pull/28878

PR: https://git.openjdk.org/jdk/pull/28878