RFR: JDK-8248863: Add feature to trigger search via URI fragment

[Hannes Wallnöfer]

Please review a new feature to trigger a javadoc search on any javadoc page through a specially encoded URI fragment. 

This allows the search feature to be triggered "locally" in the currently viewed page by clicking on a link to the search fragment, or "remotely" by linking to a page whose URI includes a search fragment. The search fragment uses the following syntax to make it differ from normal page anchors:

    #!search=search_term

Although fragments are most commonly used to target elements by their `id` attribute in HTML documents they are by no means limited to this. The [URI specification] defines the fragment in very generic terms as additional information identifying some secondary resource within the primary resource identified by the URI. There are also many examples of [proposed and existing uses] of fragment identifiers for similar tasks.

[URI specification]: https://datatracker.ietf.org/doc/html/rfc3986#section-3.5
[proposed and existing uses]: https://en.wikipedia.org/wiki/URI_fragment#Proposals

I considered other mechanisms such as query strings or javascript links, but they all have significant drawbacks compared to URI fragments. Query strings are usually associated with server side processing and cannot be opened without reloading the page. Javascript links on the other hand cannot be triggered remotely as part of the primary URI, and there are security issues associated with them. I think URI fragments are the right tool for the task.

The new feature is deployed in the [Help page] with the search examples, which should help to popularize the feature. Additionally it  should be documented in the javadoc search spec, for which I'll file a separate JBS issue. 

[Help page]: http://cr.openjdk.java.net/~hannesw/8248863/api.00/help-doc.html

Below are the URIs for the search examples in the Help page as well as a few other examples:

http://cr.openjdk.java.net/~hannesw/8248863/api.00/help-doc.html#!search=j.l.obj
http://cr.openjdk.java.net/~hannesw/8248863/api.00/help-doc.html#!search=InpStr
http://cr.openjdk.java.net/~hannesw/8248863/api.00/help-doc.html#!search=HM.cK
http://cr.openjdk.java.net/~hannesw/8248863/api.00/index.html#!search=stringbuilder.append
http://cr.openjdk.java.net/~hannesw/8248863/api.00/index.html#!search=java%20collection%20framework

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Commit messages:
 - JDK-8248863: Add feature to trigger search via URI fragment

Changes: https://git.openjdk.java.net/jdk/pull/6524/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6524&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248863
  Stats: 20 lines in 4 files changed: 16 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6524.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6524/head:pull/6524

PR: https://git.openjdk.java.net/jdk/pull/6524