Docs: cleanup of old/deprecated documents

- Remove `/development/install` page and redirect it to `/development/standards`
- Remove `/custom_installs/` folder since we don't support it and those documents are completely broken
- Remove `/development/buildenvironments` due that it's not valid anymore
- Update Front-End page to mention `inv docker.buildassets` instead
- Do not mention "Installation guide" when contributing to documentation
- Put our design documents at the bottom of the list in the index
- Put `/development/standards` at the top of the list in the index
- Remove Elasticsearch manual installation: users shouldn't use this
- Remove `MULTIPLE_APP_SERVERS` setting
- Move explanation for `DOCKER_LIMITS` settings into settings page

Author: humitos

docs/contribute.rst

@@ -25,7 +25,7 @@ Contributing to development
 
 If you want to deep dive and help out with development on Read the Docs, then
 first get the project installed locally according to the
-:doc:`Installation Guide </development/install>`. After that is done we
+:doc:`installation guide </development/standards>`. After that is done we
 suggest you have a look at tickets in our issue tracker that are labelled `Good
 First Issue`_. These are meant to be a great way to get a smooth start and
 won't put you in front of the most complex parts of the system.