docs/user/versioning: Document APIs covered by our Semantic Versioning

Also roll the tips-and-tricks docs into the overview's
multi-invocation docs, since they'd been covering the same ground with
slightly different wording before.  I've expanded the unified
description to go into a bit more detail and tie in the new versioning
docs.

I've also documented the manifest-templates target from 166a9f1e
(pkg/asset: new target manifest-templates, 2018-10-30, #592).

And I've shifted a few "target directory" references to "asset
directory", since that's the language we use for --dir (as shown by
--help).

CHANGELOG.md

@@ -138,7 +138,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - The user-facing `OPENSHIFT_INSTALL_*` environment variables are
   gone.  Instead, users who want to skip the wizard are encouraged to
   [provide their own
-  install-config](docs/user/tips-and-tricks.md#reusing-an-install-config).
+  install-config](docs/user/overview.md#multiple-invocations).
 - On AWS, the option to install a cluster into an existing VPC is
   gone.  Users who would have previously done this can use [VPC
   peering][aws-vpc-peering].