[Jan 25, 2022] Add documentation for 2FA with GitHub Mobile (#24133)

* Add documentation for 2FA with GitHub Mobile

* Fix unordered list for GitHub Mobile features

* Prefer "logging in to" over "logging into"

Co-authored-by: Brian Lovin <brianlovin@github.com>

* Indicate that GitHub Mobile is secondary

* Further indicate that GitHub Mobile is most like security keys

* Prefer "sign in" over "log in"

Co-authored-by: Zachary Sierakowski <zacharysierakowski@github.com>

* Fix unintended edit

Co-authored-by: Zachary Sierakowski <zacharysierakowski@github.com>

* Prefer active voice

Co-authored-by: hubwriter <hubwriter@github.com>

* Add missing punctuation

Co-authored-by: hubwriter <hubwriter@github.com>

* Fix phrasing of mobile 2FA CTA

Co-authored-by: hubwriter <hubwriter@github.com>

* Apply suggestions from code review

Co-authored-by: hubwriter <hubwriter@github.com>

* Prefer active voice

Co-authored-by: hubwriter <hubwriter@github.com>

* Update "secondary" phrasing for mobile 2FA

Co-authored-by: hubwriter <hubwriter@github.com>

* Add info about skipping security key

Co-authored-by: Joshua Walker <3141202+joshuawalker@users.noreply.github.com>

Co-authored-by: Brian Lovin <brianlovin@github.com>
Co-authored-by: Zachary Sierakowski <zacharysierakowski@github.com>
Co-authored-by: hubwriter <hubwriter@github.com>
Co-authored-by: Joshua Walker <3141202+joshuawalker@users.noreply.github.com>

content/get-started/using-github/github-mobile.md

@@ -21,10 +21,12 @@ redirect_from:
 {% data variables.product.prodname_mobile %} gives you a way to do high-impact work on {% data variables.product.product_name %} quickly and from anywhere. {% data variables.product.prodname_mobile %} is a safe and secure way to access your {% data variables.product.product_name %} data through a trusted, first-party client application.
 
 With {% data variables.product.prodname_mobile %} you can:
+
 - Manage, triage, and clear notifications
 - Read, review, and collaborate on issues and pull requests
 - Search for, browse, and interact with users, repositories, and organizations
 - Receive a push notification when someone mentions your username
+{% ifversion fpt or ghec %}- Secure your GitHub.com account with two-factor authentication{% endif %}
 
 For more information about notifications for {% data variables.product.prodname_mobile %}, see "[Configuring notifications](/github/managing-subscriptions-and-notifications-on-github/configuring-notifications#enabling-push-notifications-with-github-mobile)."