Add Sessions Management feature document.

[maliming]

https://github.com/abpio/abp-commercial-docs/blob/dev/en/modules/identity/session-management.md

https://github.com/abpio/abp-commercial-docs/blob/dev/en/modules/account/session-management.md

[github-actions]

Images automagically compressed by Calibre's image-actions ✨

Compression reduced images by 48.9%, saving 451.88 KB.

Filename	Before	After	Improvement	Visual comparison
en/images/my-sessions-details.png	161.17 KB	87.94 KB	-45.4%	View diff
en/images/my-sessions-list.png	157.78 KB	81.85 KB	-48.1%	View diff
en/images/my-sessions.png	65.78 KB	37.88 KB	-42.4%	View diff
en/images/prevent-concurrent-login.png	122.40 KB	55.94 KB	-54.3%	View diff
en/images/session-details-modal.png	127.51 KB	66.73 KB	-47.7%	View diff
en/images/sessions-modal.png	136.27 KB	66.20 KB	-51.4%	View diff
en/images/user-sessions.png	153.65 KB	76.13 KB	-50.4%	View diff

824 images did not require optimisation.