RFR: JDK-8263198: javadoc HELP page

[Jonathan Gibbons]

Please review an update to the structure and some of the content of the generated Help page, focussed on the following:

* A new section at the top of the Help page headed "Navigation", providing an overview of how to navigate around the docs. 
* The HELP button in the top navbar for each kind of generated page now links to the corresponding section in the help page. This implies the need for some additional sections, which are now provided.

Given other ongoing work, it is likely that there will be additional improvements to the content still to come.

We have talked about reordering the buttons in the navbar. At the same time, we might want to consider the order of sections in the page (as a separate issue.)

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Commit messages:
 - JDK-8263198: javadoc HELP page

Changes: https://git.openjdk.java.net/jdk/pull/3144/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3144&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263198
  Stats: 457 lines in 14 files changed: 312 ins; 49 del; 96 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3144.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3144/head:pull/3144

PR: https://git.openjdk.java.net/jdk/pull/3144