Simplify/document snap creation (#8404)

This PR adds the following documentation improvements to fix https://github.com/certbot/certbot/issues/7958:

- Simplify building external plugins
- Separate out certbot snap instructions from plugin instructions
- Mention that dnsimple is just an example for the plugin instructions
- Mention remote build for other architectures
- Mention snap doc exists elsewhere in developer guide (`contributing.rst`)

* Set up generate_dnsplugins_all.sh for all files and parametrize snapcraft and postrefreshhook files

* Create constraints file in the generate_dnsplugins_all script

* Separate out plugin and certbot snaps and update instructions

* Add remote build instructions

* Add pointers to the README to contributing.rst

certbot/docs/contributing.rst

@@ -375,6 +375,9 @@ The script used to generate the snapcraft.yaml files for our own externally
 snapped plugins can be found at
 https://github.com/certbot/certbot/blob/master/tools/snap/generate_dnsplugins_snapcraft.sh.
 
+For more information on building externally snapped plugins, see the section on
+:ref:`Building snaps`.
+
 Once you have created your own snap, if you have the snap file locally,
 it can be installed for use with Certbot by running:
 
@@ -534,6 +537,15 @@ Use of EFFOSCCP is subject to the `EFF Code of Conduct
 <https://www.eff.org/pages/eppcode>`_. When investigating an alleged Code of
 Conduct violation, EFF may review discussion channels or direct messages.
 
+.. _Building snaps:
+
+Building the Certbot and DNS plugin snaps
+=========================================
+
+Instructions for how to manually build and run the Certbot snap and the externally
+snapped DNS plugins that the Certbot project supplies are located in the README
+file at https://github.com/certbot/certbot/tree/master/tools/snap.
+
 Updating certbot-auto and letsencrypt-auto
 ==========================================