Add backend installation documentation.

This is an edited copy of what I included in the recent release notes.  See:
https://dist.plone.org/release/6.0.0a4/RELEASE-NOTES.md

For the moment it includes a section on how to install Volto.
This is a candidate for removal because the Volto docs are included automatically and I point to it.
But the Volto docs also talk about how to install a backend.
So maybe keep for now and improve later.

I called the main file `installation.md`.
I am fine with renaming this to `installing.md` or `install.md` if that sounds more natural to a native speaker.
I will want to point to this file from the new release notes, so best pick a name now and stick with it.

Author: mauritsvanrees

docs/backend/index.md

@@ -12,6 +12,7 @@ html_meta:
 
 ```{toctree}
 :maxdepth: 2
+installation
 control-panels
 content-types
 behaviors

docs/backend/installation.md

@@ -0,0 +1,140 @@
+---
+html_meta:
+  "description": "Install Plone 6"
+  "property=og:description": "Install Plone 6"
+  "property=og:title": "Install Plone 6"
+  "keywords": "installation, installing, buildout, pip, docker"
+---
+
+(backend-installation-label)=
+
+# Installation
+
+Some documentation about installation:
+
+* [Installation instructions from the Mastering Plone 6 training](https://training.plone.org/5/mastering-plone/installation.html)
+* {ref}`frontend-getting-started-installing-volto-label`
+* [Community post](https://community.plone.org/t/our-pip-based-development-workflow-for-plone/14562) on work in progress with [`plone-kickstarter`](https://github.com/bluedynamics/plone-kickstarter) and [`mxdev`](https://github.com/bluedynamics/mxdev).
+
+If you use Docker, we have some images:
+
+* `plone/plone-backend` (5.2 and 6.0)
+* `plone/plone-frontend` (Volto)
+* `plone/plone-haproxy`
+
+If you don't do Docker, you will have to do the backend by hand.
+The links above should give you information on how to install the prerequisites, like Python, also on Windows.
+Here, we will focus on Unix-like systems (Linux, Mac OSX), but Windows should work as well.
+The steps are:
+
+* Install the Plone (Classic) backend with buildout or pip.
+* Create the Plone Site in the browser.
+* Install the Plone frontend (Volto) with node.
+
+Below we will install Plone 6.0.0a4.
+You should use the latest from the Plone 6.0 series.
+We install it with Python 3.9.
+Python 3.7 and 3.8 are also supported.
+Please use the most recent version supported by your platform.
+
+
+(installation-backend-buildout-label)=
+
+## Install backend with buildout
+
+Change to a new directory and put a file `buildout.cfg` in it:
+
+```ini
+[buildout]
+extends = https://dist.plone.org/release/6.0.0a4/versions.cfg
+parts = instance
+
+[instance]
+recipe = plone.recipe.zope2instance
+eggs =
+    Plone
+user = admin:admin
+```
+
+Install it with:
+
+```shell
+python3.9 -m venv .
+bin/pip install -r https://dist.plone.org/release/6.0.0a4/requirements.txt
+bin/buildout
+bin/instance fg
+```
+
+
+(installation-backend-pip-label)=
+
+## Install backend with pip
+
+If you do not want to use buildout, you can install the Plone Python packages with `pip`.
+Change to a new directory and then:
+
+```shell
+python3.9 -m venv .
+bin/pip install -U pip setuptools wheel
+bin/pip install Plone -c https://dist.plone.org/release/6.0.0a4/constraints.txt
+bin/mkwsgiinstance -u admin:admin -d .
+bin/runwsgi -v etc/zope.ini
+```
+
+Note: you may need to edit `etc/zope.conf` to add a `blob-dir`.
+See [issue 3345](https://github.com/plone/Products.CMFPlone/issues/3345#issuecomment-953700024).
+
+
+(installation-create-plone-backend-label)=
+
+## Create Plone backend
+
+After you have installed the backend with buildout or pip, open a browser and go to http://localhost:8080/.
+Click 'Create a new Plone site' to prepare for the new Volto frontend.
+If you want Plone Classic instead, click 'Create Classic Plone site'.
+(If this button is not available, then you did not install `plone.volto` with buildout or pip. 'Create a new Plone site' will create a Classic site then.)
+
+Note: For Volto, make sure the Path identifier is Plone. You can change this, but then you need to change some Volto frontend configuration as well.
+
+Submit the form and your backend is ready.
+If you want Classic Plone, you are done.
+If you want the full Plone 6 with Volto, read on.
+
+
+(installation-frontend-node-label)=
+
+## Frontend with node
+
+You should probably read the chapter on {ref}`frontend-getting-started-installing-volto-label`.
+But the following gives you the general idea.
+
+First you may want to install `nvm`, the Node Version Manager:
+
+* On Linux: `apt-get install nvm`
+* On Mac: `brew install nvm`
+* Or use the installation procedure detailed in the [nvm documentation](https://github.com/nvm-sh/nvm)
+
+You use `nvm` to install recent `node/npm/yarn` versions:
+
+```shell
+nvm install --lts
+npm install --global yarn
+```
+
+Create a Volto project:
+
+```shell
+npm init yo @plone/volto
+```
+
+This will take long, and then ask for a project name.
+It will create a directory with this name.
+Go to that directory and start the frontend:
+
+```shell
+yarn start
+```
+
+In your browser go to [http://localhost:3000](http://localhost:3000/).
+
+You are done. Welcome to Plone 6!