RFR: JDK-8309471: Do not list section links in static index pages [v2]

[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.

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

  Include sections in index pages but restrict key characters

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14312/files
  - new: https://git.openjdk.org/jdk/pull/14312/files/17fc6383..9f930fa8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14312&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14312&range=00-01

  Stats: 98 lines in 4 files changed: 73 ins; 7 del; 18 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