Add a User Profile How-To to docs

[Carlton Gibson]

Following ​the forum discussion here, add a Profile How-To showing best practices for a profile model related to the User model.

• Show a best-practice model — with decent name fields.
• Be clear to tell folks that the existing first_name/last_name fields are outdated (at best) and say it’s OK to just ignore them.
• Show creating the profile instance — with a signal, inline, or using get_or_create()
• Give an introduction to template fragment caching.

[Natalia Bidart]

Gladly accepting this!

Also TIL (not really today but very recently) about template fragment caching. Looking forward to reading this!

[Abhimanyu Singh Negi]

Replying to Natalia Bidart:
Hi, I see the last owner hasn't made any progress on this ticket in the past 17 months, do you mind assigning the issue to me?

[Abhimanyu Singh Negi]

Replying to Abhimanyu Singh Negi:

Replying to Natalia Bidart:
Hi, I see the last owner hasn't made any progress on this ticket in the past 17 months, do you mind assigning the issue to me?

Raised a pr for this ticket(​https://github.com/django/django/pull/20721), kindly review.

[Natalia Bidart]

Replying to Abhimanyu Singh Negi:

Raised a pr for this ticket(​https://github.com/django/django/pull/20721), kindly review.

Hello, thank you for the PR. Please refer to the contributing docs in how to update the ticket flags so the PR appears in the review queue. I'll set it now but it would be very helpful if you would update the flags yourself in the future. Thanks again!