Move and configure lib directories

[theory]

Instead of OS-specific and Postgres version-specific lib directories, just have mod for Postgres extension module files and lib for shared libraries they depend on. For example, the pgsql_http extension's http.so file would be installed in mod and its dependency, libcurl, in lib. Both remain part of the postgres binary's runpath:

# chrpath -l $(which postgres)
/usr/lib/postgresql/bin/postgres: RUNPATH=/usr/lib/postgresql/lib:/var/lib/postgresql/tembo/mod:/var/lib/postgresql/tembo/lib

While at it, remove LLVM, Python, Tcl, and Perl from the base image, eliminating around 180MB from the image size. LLVM is not needed on the image, and the dynamic language libraries only needed if their accompanying extensions are installed; we'll want to install them when their extensions are installed.

Update docker-entrypoint.sh to add /var/lib/postgresql/tembo/mod to the dynamic_library_path, so that Postgres can find extension modules there. Control files will need $libdir/ stripped from module_pathname for this to work, a change likely to be included in Postgres 18 but we'll have to do when installing extensions there.

Other changes:

• Fix workflow push on main an move a couple of Dockerfile items around.
• Update the image URLs and directory descriptions the READMEs.