Adding an explainer, shortening the charter

[iherman]

The charter got too long and complex, and AC reps usually do not like that. I have made an attempt to:

• separate the background material into a separate explainer
• made the charter proposal way more succinct, referring to the explainer instead

This gave me the possibility to make the background material an terminology much more precise (I hope).

Here is the result:

• https://raw.githack.com/iherman/ld-signatures-charter/separate-explainer/index.html
• https://raw.githack.com/iherman/ld-signatures-charter/separate-explainer/explainer.html

It is probably still rough and will need some care, but would be good to see if the direction is feasible

cc: @msporny @phampin

[msporny]

Can we get this merged either way @iherman so I can try a PR on top of it?