DOC: internals: move some API definitions to an "api" subdirectory

It's not always easy to figure that there are some docs on internal API
stuff, let's move them to their own directory. There's a diagram for
lists that could be placed there but instead would deserve a greppable
description for quick lookups, so it was not moved there.

diff --git a/doc/internals/buffer-api.txt b/doc/internals/api/buffer-api.txt
similarity index 100%
rename from doc/internals/buffer-api.txt
rename to doc/internals/api/buffer-api.txt

diff --git a/doc/internals/filters.txt b/doc/internals/api/filters.txt
similarity index 100%
rename from doc/internals/filters.txt
rename to doc/internals/api/filters.txt

diff --git a/doc/internals/htx-api.txt b/doc/internals/api/htx-api.txt
similarity index 100%
rename from doc/internals/htx-api.txt
rename to doc/internals/api/htx-api.txt

diff --git a/doc/internals/initcalls.txt b/doc/internals/api/initcalls.txt
similarity index 100%
rename from doc/internals/initcalls.txt
rename to doc/internals/api/initcalls.txt