Remove sphinx directives from overview texts

[lengau]

What needs to get done

Like in #306 , remove sphinx directives from overview texts

Why it needs to get done

Use cases like canonical/charmcraft#2347

[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-4623.

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