RFR: 8370154: Update @jls and @jvms taglets to point to local specs dir [v3]

[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.

Dan Smith has updated the pull request incrementally with one additional commit since the last revision:

  Comment about open-source builds

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28207/files
  - new: https://git.openjdk.org/jdk/pull/28207/files/a47c3c5a..cb8de969

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28207&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28207&range=01-02

  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 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