[API docs]: Clarify Serverless "Beta" label meanings (add supportability description), and more clearly label "Technical Preview with clear supportability statements

### Description

Within the Serverless documentation:
- some pages clearly say `preview` at the top (which need to hover to see supportability statements), ie https://www.elastic.co/guide/en/serverless/current/security-add-exceptions.html

- some have the statement written out (ie https://www.elastic.co/docs/api/doc/serverless)

- some have `Beta` (with no hover or description) (ie https://www.elastic.co/docs/api/doc/serverless/operation/operation-findendpointlistitems)

It would be very helpful to add clear, consistent and formatted supportability statements for the supportability of Serverless APIs and Serverless products. Currently, the layout creates confusion about what can be supported for Serverless products and `Beta` does not appear to be explained in the Serverless documentation.

### Resources

Platform - serverless team

### Which documentation set does this change impact?

Elastic Cloud (Hosted and Serverless) only

### Feature differences

This is specific to all Serverless documentation

### What release is this request related to?

N/A

### Collaboration model

The product team

### Point of contact.

**Main contact:** @ppf2

**Stakeholders:**