[Internal]: Update CPS Docs for Observability

### Description

With the GA launch of Cross Project Search (CPS), we need to update some of the existing documentation which was prepared for Tech Preview, as some things have changed.

- [Generic CPS page](https://www.elastic.co/docs/explore-analyze/cross-project-search/cross-project-search-manage-scope#cps-availability-observability)
- [x] Remove Observability AI Assistant
- [x] SLOs - change to _Available (scope selector available)_
- [x] Streams - TBC
- let's get input from the Streams team on this [DONE - see below]
- [x] [Observability CPS page](https://www.elastic.co/docs/solutions/observability/cross-project-search#obs-cps-compatibility)
- [x] Repeat as above to make consistent with the above
- [SLO section](https://www.elastic.co/docs/solutions/observability/cross-project-search#obs-cps-slo-remote)
- @kdelemme is there anything specific to call out here, or remove this?
- [x] [Default data view](https://www.elastic.co/docs/solutions/observability/cross-project-search#obs-cps-no-data-views)
- No changes are being made here now (as per the linked https://github.com/elastic/kibana/issues/260930 issue) - either remove this section, or if you think it's clearer, leave it there but remove the Kibana link (as it suggests this is something that's going to be _fixed_) [removed]

* [x] Note that the CPS switcher is changing (to allow individual projects to be selected, as opposed to _all or nothing_, so [this section](https://www.elastic.co/docs/solutions/observability/cross-project-search#obs-cps-scope-selector) should probably be updated to more generally describe the behaviour. (fixed in https://github.com/elastic/docs-content-internal/issues/1497)

### Resources

The [#cps-project-team](https://elastic.slack.com/archives/C08FLNBAPUG) slack channel

### Which deployment methods does this change impact?

Elastic Cloud Serverless only

### Feature differences

_No response_

### What Elastic Stack release is this request related to?

N/A

### Serverless release

_No response_

### Collaboration model

The documentation team will create the first draft

### Point of contact.

**Main contact:**
- @paulb-elastic

**Stakeholders:**
- @vinaychandrasekhar
- @tiamliu