RFR: JDK-8248863: Add search landing page to API documentation

[Hannes Wallnöfer]

This is the second of two PRs to enhance JavaDoc search, it is based on the first one (#8185).

It adds a standalone search page (search.html) along with its own script file (search-page.js). This PR is very similar to the last prototype I uploaded and demoed, the changes are mostly tweaks to the markup, style sheets and text. 

JDK API docs rendered with this patch can be viewed and tested here (top-level files only):
http://cr.openjdk.java.net/~hannesw/8248863/api.01/

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Depends on: https://git.openjdk.java.net/jdk/pull/8185

Commit messages:
 - JDK-8248863: Add search landing page to API documentation

Changes: https://git.openjdk.java.net/jdk/pull/8226/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8226&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248863
  Stats: 739 lines in 20 files changed: 678 ins; 7 del; 54 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8226.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8226/head:pull/8226

PR: https://git.openjdk.java.net/jdk/pull/8226