RFR: 8335911: Document ccls indexer in doc/ide.md

[Volker Simonis]

`doc/ide.md` should also mention the [ccls indexer](https://github.com/MaskRay/ccls/wiki/Visual-Studio-Code).

Project files for it can be created by `make vscode-project-ccls`.

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Commit messages:
 - 8335911: Document ccls indexer in doc/ide.md

Changes: https://git.openjdk.org/jdk/pull/20082/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20082&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335911
  Stats: 8 lines in 2 files changed: 2 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/20082.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20082/head:pull/20082

PR: https://git.openjdk.org/jdk/pull/20082