Add contextual help to BP Email WP Admin screens

[imath]

Following [13852] & [13853], we decided on may 16th (2024) to add help tabs linking to the user documentation page explaining BP Emails management & BP Email tokens.

• Improve bp_core_add_contextual_help() to avoid code duplication.
• Add an help tab linking to the user documentation page explaining how to manage BP Emails.
• Add an help tab linking to the user documentation page explaining BP Email tokens usage.
• Deprecate bp_admin_email_add_codex_notice().

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/9159

[imath]

In 13868:

Add Help tabs to inform about BP Emails management & BP Email tokens

• Improve bp_core_add_contextual_help() to avoid code duplication.
• Add an help tab linking to the user documentation page explaining how to manage BP Emails.
• Add an help tab linking to the user documentation page explaining BP Email tokens usage.
• Deprecate bp_admin_email_add_codex_notice().

Fixes #9159
Closes ​https://github.com/buddypress/buddypress/pull/291