Streamline safe extensions section

[bifurcation]

As discussed in #34, the additional protocol mechanisms defined in the "Extension Designer Tools" section are not necessary for safe extensions. We can discuss separately whether they should be re-added; my argument would be that they are not necessary, in that they don't really add anything over the extensibility mechanisms.

Fixes #34

[raphaelrobert]

@kkohbrok I think the original text was from you.

[rohanmahy]

If we remove a bunch of this text, there is text missing about the consequences (now the stack or application need to be configured with a knowledge of what is safe and in what scope). Suggested text:

"In some cases, a group or related extensions (often of multiple types) may be used only together as a single "feature". It is appropriate to define an additional LeafNode or GroupContext extension that means support for a specific combination of extensions. In order to determine if a single extension is safe, or if a combination of extensions in a single "feature" can safely access the same state, the stack or application need to be configured with that information. Misconfiguration could result in a catastrophic violation of the safe extension contract."