Add support for new applies_to value (stateless)

A new stateless tag is required for `stateless` to represent a new flavor of our core Elastic platform (as per our [structured model](https://elastic.github.io/docs-builder/syntax/applies/#structured-model)).

Related docs issue: https://github.com/elastic/docs-content-internal/issues/475
Related eng issue: https://github.com/elastic/roadmap/issues/104

Please let me know what other information you'd need.
Many thanks!

Admin docs context (SR)
Today's `applies_to` model has slots for [deployment types](https://www.elastic.co/docs/contribute-docs/how-to/cumulative-docs/reference) (`ech`, `ece`, `eck`, and `self`) and `serverless` separately. Stateless is a new flavor that will surface across self-managed and ECK (admin docs tracking: [docs-content-internal#475](https://github.com/elastic/docs-content-internal/issues/475)). Without a tag, the [deployment type audit pipeline](https://github.com/elastic/docs-content-internal/issues/1359) can't flag missing or incorrect Stateless applicability on admin pages.
## What's needed
A way to tag content that applies to Stateless deployments wherever they run.
## Open questions
- Schema design: new top-level key (`stateless`), sub-key under `deployment`, value under existing dimensions, or something else
- How stateless interacts with the existing `stack` and `serverless` dimensions (Stateless self-managed and ECK look more like stack than serverless, but architecturally they're closer to serverless)
- Migration of any existing stateless content
## Outcomes
- New tag supported in `applies_to`
- [Cumulative docs reference](https://www.elastic.co/docs/contribute-docs/how-to/cumulative-docs/reference) and [deployment types guide](https://www.elastic.co/docs/contribute-docs/how-to/deployment-types) updated
- [`docs-applies-to-tagging` skill](https://github.com/elastic/elastic-docs-skills/tree/main/skills/authoring/applies-to-tagging) updated
- Existing pages migrated where applicable