docs/5_Component_Documentation/1_agl-compositor: Slash headers

docs/5_Component_Documentation/1_agl-compositor: Slash headers

mkdocs doesn't generate links for Header 3 types, so use Header 2
to have more links available, in case these might be needed.

Change-Id: I33c5e0bf5bcd74835f49ca7cd599753335b015ac
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/26668
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>