Manage Plans section

We need to add information to the documentation for Plans so that labels can be assigned to plans, allowing plans to be grouped accordingly.
Also add link to Labels page https://docs.testomat.io/advanced/tags-labels/labels-and-custom-fields/

<img width="2810" height="1450" alt="Image" src="https://github.com/user-attachments/assets/0b3d2742-55e5-42d4-87b2-e9553922a2c0" />