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
