User guide docs: how to use ApplicationSet controller, whats/whys/hows, examples

[jgwest]

The sections are currently out of order in the docs (this will be fixed by #115), but this is the order in which they will appear:

• Introduction
• Use Cases
• How ApplicationSet controller interacts with Argo CD
• Generators
• Template fields
• Application Pruning & Resource Deletion

You can see the docs (via GitHub Markdown rendering) here: https://github.com/jgwest/applicationset/tree/w3-user-guide-84/docs

Caveats:

• Install instructions won't work until we have pushed our first image (hopefully soon)
• The examples won't yet work until this PR merges (I tested them on my local branch, before I renamed all the references for this PR).

Future work:

• As mentioned above, I will be adding additional mkdocs/readthedocs integration with this issue.
• I hope to have our technical writing folks have a look at this, but I would like to merge this PR first.
• This doc does not include documentation on the following 'advanced' topics, or topics on newly delivered features, which will be covered by future issues:
  • Application naming strategies
  • Multiple generators specified in the generators field
  • Values in list/cluster generators
  • Application Sync strategies
  • The Sync Policy parameter of controller
• Add README.md files to the individual examples under examples

In order to ensure consistency throughout the document, I created an arbitrary style guide:

• Use "ApplicationSet resource" to refer to the ApplicationSet CR within a Kubernetes namespace, otherwise just use "ApplicationSet". Likewise use "Application resource" for Argo CD Applications.
• The name of generator is always capitalized (Git, List, Cluster, generators)
• It's "Argo CD", not "ArgoCD" (as per Argo CD docs)
• Don't use 'repo' as short for repository, or 'appset' as short for ApplicationSet, just use the full name
• Use 'ApplicationSet' not 'Application Set'
• Use 'Argo CD Application' or 'Application', to refer to Argo CD Applications.
• "multitenant", not "multi-tenant"
• As is often the case with technical docs, written in impersonal third person
• Serial commas 😛

Fixes #84
Fixes #85

[wtam2018]

LGTM
Thanks @jgwest