add link to new publisher registration form

[KDuerden]

Update to change method of registering with 360Giving from emailing support@ to link to new registration form.

[KDuerden]

This is PR to make changes to guidance for publishers parts - which are in the non-normative parts of the standard docs site. Therefore it can be checked, approved and merged without reference to the change management governance processes.

[KDuerden]

On before starting I noticed an unrelated typo:

"You will decided" should be "You will decide"

I've fixed that now thank you.

[KDuerden]

@mrshll1001 are you ok to give a final check and merge if you agree it's ready?