docs: document login_redirect filter for post-2FA redirect (#278)

[masteradhoc]

Fixes #278

What?

Adds a short documentation entry to readme.txt explaining how to redirect users after the two-factor challenge completes.

Why?

There was no documentation on how to control the post-login destination when 2FA is enabled.

How?

Adds a new = Redirect After the Two-Factor Challenge = entry to the Actions & Filters section in readme.txt with a working login_redirect filter example.

Testing Instructions

1. Install the plugin and enable 2FA for a user.
2. Add the following to functions.php:

add_filter( 'login_redirect', function( $redirect_to, $requested_redirect_to, $user ) {
    return home_url( '/dashboard/' );
}, 10, 3 );

3. Log in with that user and complete the 2FA challenge.
4. Confirm the user lands on /dashboard/.

Screenshots or screencast

N/A

Changelog Entry

Added - Document the login_redirect filter for controlling redirect destination after the two-factor challenge.

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Core SVN

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Props: masteradhoc, kasparsd.

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Co-authored-by: masteradhoc <masteradhoc@git.wordpress.org>
Co-authored-by: kasparsd <kasparsd@git.wordpress.org>

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[kasparsd]

Should we call out that this is a core WP filter and the plugin doesn't require any special treatment of the login redirect?

[masteradhoc]

@kasparsd good point, done :) what do you think?