docs/develop: reorganize app_server documentation

Separate the "interface specification" into multiple individual, shorter
pages. Also update some outdated parts and replace references to
OpenBeOS w

docs/develop: reorganize app_server documentation

Separate the "interface specification" into multiple individual, shorter
pages. Also update some outdated parts and replace references to
OpenBeOS with Haiku.

Change-Id: I86baa9a78c5f7a8a3c4c7aa3ac2992714afecae6
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6084
Tested-by: Automation <automation@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

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