Documentation: document ability to control the `next` parameter when authenticating

[Stormheg]

Follow-up on #78 (comment)

What

A next_field_selector parameter to the render_otp_webauthn_auth_scripts can be used to control which field the JavaScript will use to determine where to redirect to upon completing Passkey authentication.

Example snippet

The next_field_selector must be a CSS selector.

<input type="hidden" name="volgende" value="/url-to-redirect-after-login">

{% render_otp_webauthn_auth_scripts next_field_selector="input[name='volgende']" %}

cc @activus-d