docs/develop/ide: A quick guide for haiku code completion

* Works under a Linux "cross-compile" environment
* We can't check in compile_commands.json unforunetly because it
highly depends on your build environment
* An in-tree .clangd is a thing, but our includes are complex
and directory-spefific.
* Thanks to Pulkomandy for adding the flag to jam to do this!

Change-Id: I3be4084c43f7b822bb04ea7ec527c5fbe03d7289
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7158
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>