RFR: JDK-8309471: Do not list section links in static index pages

[Hannes Wallnöfer]

Please review a simple change to avoid adding section index items (added in JDK-8286470) from appearing in the static index pages where the tend to cause trouble. 

The change is implemented by making `IndexItem.isTagItem()` behave true to its specification again and only return `true` if it actually represents a JavaDoc tag, and add a new `IndexItem.isSectionItem()` method for the new section index items. The new method is used to make sure section links are not listed in the index pages.

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

Commit messages:
 - Undo unintentional reformatting
 - JDK-8309471: Do not list section links in static index pages

Changes: https://git.openjdk.org/jdk/pull/14312/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14312&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309471
  Stats: 25 lines in 3 files changed: 21 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14312.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14312/head:pull/14312

PR: https://git.openjdk.org/jdk/pull/14312