Add TL;DR section at the top of every manual page

Description

Like it or not, users often skim the docs, and the most important points should be prominent.

Example of a question I got recently: what's the default for speculative executions? From someone who had "read" that page.

One-sentence description of the feature + the main takeaway points (default value, possible alternatives, etc.)

Environment

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Status

Assignee

Olivier Michallat

Reporter

Olivier Michallat

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Fix versions

Priority

Minor
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