Shift focus away from "login" to "explore" for new users

[waxlamp]

Motivation: stop new users from falling into the rabbit hole of creating an account before making use of the archive. This avoids the frustration of users giving up in the middle of creating the account (leaving it INCOMPLETE) or having to be rejected by an admin at cost to the admin team (leaving it REJECTED and requiring an appeal later, if and when they do need the account).

Solution:

• make the login button neutral colored instead of blue
• add a blurb below the intro text explaining that all data can be examined and used WITHOUT registering an account
• add a blue button that takes the user to the public dandiset listing

Here's a (very bad) mockup of what I'm picturing:

The overall effect is to move the "call to action" from the login / create account button to an explore button.

This is intended to solve the fundamental problem addressed by #2038 (and others).