RFR: JDK-8248863: Add search landing page to API documentation [v2]

[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/

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  First round of feedback

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8226/files
  - new: https://git.openjdk.java.net/jdk/pull/8226/files/089e12ea..b47c7346

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8226&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8226&range=00-01

  Stats: 18 lines in 6 files changed: 5 ins; 6 del; 7 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