Schema definition for external tooling (VS Code extension support)

The VS Code Extension for V3 docs is about to be open sourced: https://github.com/elastic/elastic-docs-v3-vscode. The main issue with it is how to keep it updated with V3 adding or removing stuff, especially in the frontmatter and directives.

I see opportunities for schema definition and content reuse in the docs-builder project that could allow the VS Code extension and any kind of tool to obtain quick, structured knowledge of the V3 syntax:

## Directive schema

**Current state:**
- 13+ directive types implemented in C# (`src/Elastic.Markdown/Myst/Directives/`)
- Directives include: Admonition, AppliesSwitch, AppliesTo, CsvInclude, Diagram, Dropdown, Image, Include, Mermaid, Settings, Stepper, Tabs, Version

**Opportunity:**
Create a **JSON Schema for Directive Definitions** that can be:
- Consumed by VS Code extensions for autocompletion
- Used for validation in CI/CD pipelines
- Shared across documentation teams
- Version-controlled separately from implementation

## Configuration schema

### docset.yml schema definition

**Current state:**
- YAML configuration file with fields: project, max_toc_depth, dev_docs, cross_links, exclude, subs, features, api, toc
- No formal JSON Schema for validation

**Opportunity:**
Create a **JSON Schema for docset.yml** to enable:
- IDE autocompletion and validation
- Pre-build validation
- Documentation generation
- Type-safe configuration

### Frontmatter schema

**Current state:**
- 4 documented frontmatter fields: navigation_title, description, applies_to, products
- No formal schema definition

**Opportunity:**
Create a **Frontmatter JSON Schema** with:
- All supported fields
- Type validation
- Product ID enumeration
- applies_to validation

### applies_to schema

**Current state:**
- Complex nested structure with multiple keys (stack, deployment, serverless, product)
- Lifecycle states: preview, ga, beta, deprecated, removed
- Version specifications

**Opportunity:**
Create a dedicated **applies_to JSON Schema** for reuse across:
- Frontmatter validation
- Directive parameter validation
- Build-time validation

## Content reuse

### Substitution variables as shared schema

**Current state:**
- Substitution variables defined in docset.yml (`subs:` key)
- 200+ variables in example docset.yml
- Variables include: product names, app names, URLs, feature names
- Support for mutations (lowercase, uppercase, kebab-case, etc.)

**Opportunity 1: central variable registry**
Create a **shared substitution variables registry** as JSON:

**Benefits:**
- Single source of truth for common variables
- Can be imported into individual docset.yml files
- Version control for variable changes
- Autocompletion support in IDEs