lang/erlang: Fix documentation chunks build and install

[dumbbell]

Summary

Currently, the CHUNKS option is ineffective because chunks are never installed. Also, running make all docs in a single make(1) breaks with Erlang/OTP 26.

This patch splits the build of all and docs into two commands and install docs after the main files. It also uses the DOC_TARGETS make variable to configure which type(s) of documentation to build and install. This changes how FOP and PDF are handled.

Test Plan

poudriere testport (...) lang/erlang

[dumbbell]

Remove .if conditions around docs build and install. The DOCS option is not defined/used in this port.

[dch]

after building & installing, I still don't see any inline docs via:

$ erl
> l(httpc).
> h(httpc).
...

for example. Are we expecting them to show up anywhere else I should check?