[Improvement] <onboarding> Improve the hint when user creating their user name

[EiffelFly]

Is There an Existing Issue for This?

• [X] I have searched the existing issues

Where are you aiming to make improvements?

Instill Core

Is your Proposal Related to a Problem?

Users got struck at the onboarding form and misunderstand that the username input is a input they need to fill in

Describe Your Proposed Improvement

• Remove the instill.tech namespace
• Make it act like GitHub way of doing things, hint user the full URL like https://instill.tech/<your_username>

Highlight the Benefits

Clear Hint, better user experience

Anything Else?

No response