RFR: 8268978: Document the javadoc software stack

[Jonathan Gibbons]

Please review a `noreg-doc` update to document the javadoc software stack, in the top-level `jdk.javadoc.internal` package.

The intent is to provide a broad overview.   We can add more specific details, but in general, additional details would probably be better in the `package-info` files for the constituent packages.

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Commit messages:
 - 8268978: Document the javadoc software stack

Changes: https://git.openjdk.java.net/jdk/pull/6914/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6914&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268978
  Stats: 128 lines in 1 file changed: 121 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6914.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6914/head:pull/6914

PR: https://git.openjdk.java.net/jdk/pull/6914