Change the introduction, adds a new section, memory to Simon.

In this PR, the following is changed:

introduction first sentence changed to more normative style,
ECH is described as both positive and creates issues,
this document is not looking for a solution nor is questioning ECH,
a new section on why the SNI is used by middleboxes was started as experimental,
an editorial fix in the contribution of Gianpaolo was fixed,
an acknowledgment to the memory of Simon was added.

echdeploy / draft-ech-deployment-considerations