Improvements for Documentation page

[GitGab19]

In current docs page there are the new roles and sub-protocols introduced by SV2: if the roles can be well understood looking at homepage schemas, maybe the new sub-protocols are not very understandable without any schema.

For this reason I worked on a draft proposal, which restructures the layout of that page, inserting the schema present in the homepage, customized for every role and sub-protocol.

For example, this is the improvement which I thought for the Roles section:

NOTE: copy of Job Negotiators and Template Providers need to be reviewed, since I think that the words I put in red have to be deleted/changed in some way.

This is the improvement for the Sub-protocols section:

NOTE: here I'd like to have opinions about the Job Distribution Protocol, since I don't know if it could be better to not mention it (in a first place) since it's not well described in specs.

Warning For now I just worked on Canva, so I need to think how to implement it yet. Also I think that the schemas which I modified, have to be redesigned in a more accurate way, if they are checked and approved by all of you.

Hope to get as more feedback as possible!

[GitGab19]

In case you think that graphical clarifications about the types of channel could be needed, I drafted a section also for them.