feat: telegraf-controller scaffolding

[sanderson]

• Base changes for the introduction of Telegraf Controller
• Leaves page templates and TODOs for Telegraf controller docs
• Updates dropdown nav to include Telegraf Controller
• Updates logic for including the platform reference in the side nav.

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• Rebased/mergeable

[Copilot]

Pull request overview

This PR introduces scaffolding for Telegraf Controller, a new enterprise product for managing Telegraf deployments at scale. The changes establish the foundational structure for Telegraf Controller documentation while marking it as a private alpha product with appropriate banners and navigation integration.

Key changes:

• Adds Telegraf Controller to the product selector dropdown with alpha state badge
• Creates placeholder documentation pages with frontmatter and TODO markers for future content
• Implements an alpha state banner with sign-up CTA for the private alpha program
• Refactors sidebar platform menu logic from regex to explicit menu key whitelist

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
layouts/partials/topnav/product-selector.html	Adds Telegraf Controller to product definitions and creates separate "Telegraf" product group with both Telegraf v1 and Controller (alpha)
layouts/partials/sidebar.html	Refactors platform menu whitelist from regex pattern to explicit slice of menu keys for better maintainability
layouts/partials/article/special-state.html	Updates special state banner to display alpha messaging for Telegraf Controller with sign-up CTA and community feedback channels
data/products.yml	Adds telegraf_controller product definition with basic metadata (commented AI fields for future implementation)
content/telegraf/controller/telegraf-enterprise.md	Creates placeholder page for Telegraf Enterprise overview with TODO comment
content/telegraf/controller/labels/_index.md	Creates placeholder section index for managing labels with cascade draft state
content/telegraf/controller/install.md	Creates placeholder page for installation instructions with TODO comment
content/telegraf/controller/configs/_index.md	Creates placeholder section index for managing configurations with cascade draft state
content/telegraf/controller/agents/_index.md	Creates placeholder section index for managing agents with cascade draft state
content/telegraf/controller/_index.md	Creates root documentation page for Telegraf Controller with TODO comment
assets/styles/layouts/article/blocks/_special-state.scss	Adds button styling for CTA in special state banners using gradient theme
PLATFORM_REFERENCE.md	Updates Telegraf documentation link from v1.36 to v1.37