[Meta] Add serverless feature tier content to our licensing docs

[florent-leborgne]

• This page/section should address serverless feature tiers as well to help users understand overall how it works. More targeted subscription clarification for serverless #7129

• In a general manner, we should check our various doc sets (mainly thinking about deploye-manage and solutions) to make sure that the difference between subscription level on Elastic Cloud and serverless feature tiers are clear and what each does for each type of instance.

More context in the parent issue

core page in question https://www.elastic.co/docs/deploy-manage/license

[shainaraskas]

core issue fixed by #7129

additional fixes needed to clean up other pages