template: simplify the docs/ scaffolding a bit

- remove the (mostly) useless sphinx-dev env
- install swh.docs in non-editable mode, and
- use the Makefiles provided by swh.docs as non-package data (in share/)
  instead of depending on the swh.docs sources to be available somewhere
  nearby.

copier-template/docs/Makefile

-include ../../swh-docs/Makefile.sphinx
+include swh-docs/Makefile.sphinx

copier-template/tox.ini.jinja

@@ -55,27 +55,11 @@ allowlist_externals = make
 extras =
   testing
 deps =
-  # fetch and install swh-docs in develop mode
-  -e git+https://gitlab.softwareheritage.org/swh/devel/swh-docs.git\#egg=swh.docs
+  # fetch and install swh-docs
+  git+https://gitlab.softwareheritage.org/swh/devel/swh-docs.git\#egg=swh.docs
 setenv =
   SWH_PACKAGE_DOC_TOX_BUILD = 1
   # turn warnings into errors
   SPHINXOPTS = -W
 commands =
-  make -I ../.tox/sphinx/src/swh-docs/swh/ -C docs
-
-# build documentation only inside swh-environment using local state
-# of swh-docs package
-[testenv:sphinx-dev]
-allowlist_externals = make
-extras =
-  testing
-deps =
-  # install swh-docs in develop mode
-  -e ../swh-docs
-setenv =
-  SWH_PACKAGE_DOC_TOX_BUILD = 1
-  # turn warnings into errors
-  SPHINXOPTS = -W
-commands =
-  make -I ../.tox/sphinx-dev/src/swh-docs/swh/ -C docs
+  make -I {env_dir}/share/ -C docs