Update the Storage service plan user guide with detailed steps to create a new storage service plan

[anvithks]

Issue/Feature Description:
In the Jerba release of SODA we have released the Storage Service Plan feature for multicloud.
Since this is a new feature users will need to know exactly who can use it and how they can use it.
The current user guide has the basic steps and covers the overall flow of using Service plans but there are a few steps which are missing.

• Add Tenants to a service plan
• Add backends to a service plan
• Remove tenants and the conditions in which they cannot be removed
• Remove backends and the conditions in which they cannot be removed.

Why this issue to fixed / feature is needed(give scenarios or use cases):
For completeness of the User Guide.

How to reproduce, in case of a bug:

Other Notes / Environment Information: (Please give the env information, log link or any useful information for this issue)