Shorten and reorganize "Avoiding key reuse" text

In the "Avoiding Key Reuse" part, I've tried to address the following issues (supposing we agree these are indeed issues):

- As suggested by Mike, putting too much emphasis on key reuse as being a security problem might raises some disagreements.
- "Security considerations" text actually ends with text that is related to key reuse, but not security.
- The text is pretty long compared to other subsections and my impression is that some parts are repeating.

As a suggestion to fix those issues, I've made the following changes:

- Add an introduction text, summarizing the problems and the solutions right from the start.
- Try to shorten texts or cut repeating texts, so privacy / security texts have equivalent lengths.
- Drop the "Security considerations" title.

The resulting texts is organized as follow:

- Introduction to the problems and the solution
- Privacy: Details about the problem how key rotation fix it.
- Security: Details about potential security risks and how key rotation is more secure.
- Explaining the only valid reasons / cases where keys are reused.
- Conclusion and recommendations