docs migration: Container Lifecycle Hooks into Concepts (#2564)

* initial commit for structure

* migrate Container Lifecycle Hooks and related files to Concepts

* update Concepts ToC

* fix formatting

* Formatting tweaks

* comment out What's next

* remove What's next

* Begin rewrite of container-lifecycle-hooks.md

* incremental update to edit

* Split Container Lifecycle Hooks into container-lifecycle-hooks.md and container-environment.md; change all referring links.

* Add concept template to volumes.md and images.md

* Apply changes for steveperry-53 feedback.

* fix format issue

* tweak formatting

* fix Concepts ToC

* update links

* add back changes to Concepts ToC

* fix a link

* update links

_data/concepts.yml

@@ -19,6 +19,7 @@ toc:
 - title: Containers
   section:
   - docs/concepts/containers/images.md
+  - docs/concepts/containers/container-environment-variables.md
   - docs/concepts/containers/container-lifecycle-hooks.md
 
 - title: Workloads