Update "Backend installation details" section

Author: stevepiercy

docs/install/manage-add-ons-packages.md

@@ -342,7 +342,7 @@ Follow the upgrade information, if present.
 
 ### Build and start your instance
 
-Build and run Plone backend.
+Build and run the Plone backend.
 
 ```shell
 make build-backend
@@ -358,33 +358,49 @@ Your instance is running in the foreground.
 
 ## Backend installation details
 
-Inside your project, open `backend/Makefile`.
+The `Makefile` at the root of you project invokes commands in `backend/Makefile`.
+Here are excerpts from `backend/Makefile` to show details of the `make build-backend` command.
 
 ```makefile
 bin/pip:
-  @echo "$(GREEN)==> Setup Virtual Env$(RESET)"
-  python -m venv .
-  bin/pip install -U "pip" "wheel" "cookiecutter" "mxdev"
+	@echo "$(GREEN)==> Setup Virtual Env$(RESET)"
+	python3 -m venv .
+	bin/pip install -U "pip" "wheel" "cookiecutter" "mxdev"
 
-instance/etc/zope.ini:  bin/pip
-  @echo "$(GREEN)==> Install Plone and create instance$(RESET)"
-  bin/cookiecutter -f --no-input --config-file instance.yaml https://github.com/plone/cookiecutter-zope-instance
-  mkdir -p var/{filestorage,blobstorage,cache,log}
+instance/etc/zope.ini:	bin/pip
+	@echo "$(GREEN)==> Install Plone and create instance$(RESET)"
+	bin/cookiecutter -f --no-input --config-file instance.yaml https://github.com/plone/cookiecutter-zope-instance
+	mkdir -p var/{filestorage,blobstorage,cache,log}
 
+# ...
+
+.PHONY: build-dev
 build-dev: instance/etc/zope.ini ## pip install Plone packages
-  @echo "$(GREEN)==> Setup Build$(RESET)"
-  bin/mxdev -c mx.ini
-  bin/pip install -r requirements-mxdev.txt
+	@echo "$(GREEN)==> Setup Build$(RESET)"
+	bin/mxdev -c mx.ini
+	bin/pip install -r requirements-mxdev.txt
 ```
 
-/ `make build-backend`
+The command `make build-backend`:
+
+-   Invokes the target `build-dev` target in `backend/Makefile`.
+-   This invokes the target `instance/etc/zope.ini`.
+-   This invokes the target `bin/pip`.
+
+    -   This creates a `Python` virtual environment if one does not exist.
+    -   It installs and upgrades Python package management tools in that virtual environment.
+
+-   Returning to the target `instance/etc/zope.ini`:
 
--   Creates a `Python` virtual environment if one does not exist, then upgrades Python package management tools.
--   Creates or updates the Zope configuration from its `instance.yaml` file using `cookiecutter-zope-instance`.
--   Creates specified directories, if they do not exist.
--   Installs Plone core packages and add-ons according `mx.ini`, `requirements.txt` and `constraint.txt`
+    -   This creates or updates the Zope configuration from its `instance.yaml` file using `cookiecutter-zope-instance`.
+    -   Creates specified directories, if they do not exist.
 
-You can configure your Zope instance as described in section {ref}`manage-common-management-tasks-label`.
+-   Returning to the target `build-dev`:
+
+    -   This generates the `mxdev` files as described above in {ref}`manage-mxdev-usage-overview-label`.
+    -   Installs Plone core packages and add-ons from the files generated by `mxdev`.
+
+You can configure your Zope instance as described in the section {ref}`manage-common-management-tasks-label`.
 
 
 ````{todo}
@@ -395,6 +411,9 @@ configuration frontend:
 
 Add info here or link to respective frontend chapter.
 localhost:8050/volto/addons/index.html#configuring-a-volto-project-to-use-an-addon
+This already exists above:
+For developing add-ons for the Plone frontend, Volto, see {doc}`volto/addons/index`.
+
 
 run
 ```