[lldb][AArch64] Add release notes and documentation for SME (#66767)

This adds a release note for all the SME support now in LLDB and a page
where I have documented the user experience (for want of a better term)
when using SVE and SME.

This includes things like which mode transitions can or cannot be
triggered from within LLDB. I hope this will serve to A: document what
I've implemented and B: be a user's guide to these extensions.

(though it is not a design document, read the commits and code for that
sort of detail)