RFR: 8370154: Update @jls and @jvms taglets to point to local specs dir

[Dan Smith]

Updated the JSpec taglets to use relative links into `../specs/jls` and `../specs/jvms` rather than external links, possible now due to JDK-8370153.

I was surprised there doesn't seem to be a standard way to get a relative path to the docs root, but I copy/pasted from ToolGuide, and made some tweaks based on failures I saw.

Tested with 'make docs' and manually confirmed that a handful of different `@jls` and `@jvms` tags are pointing to the right place.

-------------

Commit messages:
 - 8370154: Update @jls and @jvms taglets to point to local specs dir

Changes: https://git.openjdk.org/jdk/pull/28207/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28207&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370154
  Stats: 59 lines in 2 files changed: 38 ins; 14 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/28207.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28207/head:pull/28207

PR: https://git.openjdk.org/jdk/pull/28207