feature #7804 Rewriting the service container docs (weaverryan)

This PR was squashed before being merged into the 2.7 branch (closes #7804).

Discussion
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Rewriting the service container docs

Before updating the DI for 3.3, I wanted to rewrite some of the docs on 2.7.

Commits
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d76a5e8 Rewriting the service container docs

Author: weaverryan