Add extension hook points to all special usecases to inject themselves into docs build process

[Mpdreamz]

Implements detection-rules as an extension.

Extensions implement IDocsBuilderExtension and allows us to build specific use case extensions for teams by hooking into various places (TOC building/File scanning/rendering) while not injecting too much bespoke logic in our common building blocks.

Extensions are opt-in and can be enabled in docset.yml e.g:

extensions:
  - detection-rules

This particular one allows you to use a special detection_rules instruction to discover TOML detection rule set definition files as children.

toc:
  - file: index.md
    detection_rules: '../rules'

[Copilot]

PR Overview

This PR implements a new extension system for detection rules, moves configuration management for enabled extensions, and updates documentation to reflect these changes.

• Introduces a DetectionRuleFile class and corresponding parser for detection rule TOML files.
• Extracts extension handling into a dedicated EnabledExtensions class and registers new documentation builder extensions.
• Updates navigation, configuration file parsing, and file scanning to integrate detection rules support.

Reviewed Changes

File	Description
src/Elastic.Markdown/Extensions/DetectionRules/DetectionRuleFile.cs	Adds detection rule file parsing and markdown generation for detection rules.
src/Elastic.Markdown/Extensions/DetectionRules/DetectionRule.cs	Sets up the DetectionRule record and its TOML parser with required metadata.
src/Elastic.Markdown/IO/Configuration/EnabledExtensions.cs	Introduces an extension configuration wrapper which handles enabled extensions.
docs/configure/content-set/extensions.md	Updates documentation with configuration sample for detection rules extension.
Other files	Integrates detection rules extensions into documentation navigation, file scanning, and build context.

Copilot reviewed 20 out of 20 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/Elastic.Markdown/IO/Configuration/ConfigurationFile.cs:189

• In the 'detection_rules' case of the ReadChildren method, the code appends '/{folder}' to the parentPath even though 'folder' is not guaranteed to have been initialized in this branch. Consider using a variable that holds the detection rules folder name or verify that 'folder' is set.

parentPath += $"/{folder}";