Add Nix flakes to Dependabot supported ecosystems

[JamieMagee]

Why:

Towards dependabot/dependabot-core#7340

Dependabot version updates now support Nix flakes (changelog, dependabot-core#14498). The docs need to list nix as a supported package ecosystem.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Adds nix to the Dependabot ecosystem tables and creates a feature flag for version gating.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Source	Review	Production	What Changed
code-security/reference/supply-chain-security/dependabot-options-reference.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14
code-security/reference/supply-chain-security/supported-ecosystems-and-repositories.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	from reusable

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[Copilot]

Pull request overview

Updates the Dependabot documentation to include the newly supported Nix flakes ecosystem (nix) and gates the new content behind a feature flag so it only appears for supported product versions.

Changes:

• Adds nix to the “supported package managers”/ecosystems tables (behind {% ifversion dependabot-nix-support %}).
• Documents Nix flake behavior/limitations in the supported package managers reusable.
• Introduces a new feature-flag definition (dependabot-nix-support) for versioned content gating.

Show a summary per file

File	Description
data/reusables/dependabot/supported-package-managers.md	Adds a gated table row for nix and a new “Nix” section describing behavior/limitations.
data/features/dependabot-nix-support.yml	Defines the dependabot-nix-support feature flag used for version gating.
content/code-security/reference/supply-chain-security/dependabot-options-reference.md	Adds a gated nix row to the package-ecosystem table and includes minor formatting fixes.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 3

[Copilot]

Pull request overview

Updates the Dependabot documentation to include Nix flake (nix) support, gated behind a new feature flag so the ecosystem only appears for applicable product versions.

Changes:

• Add nix/Nix to the supported package managers table (with capability matrix) and add a new “Nix” ecosystem details section.
• Add a new dependabot-nix-support feature flag for version gating.
• Add nix (“Nix flakes”) to the package-ecosystem reference table in the Dependabot options reference.

Show a summary per file

File	Description
data/reusables/dependabot/supported-package-managers.md	Adds a version-gated “Nix” row to the supported ecosystems table and a new version-gated “### Nix” section describing behavior/limitations.
data/features/dependabot-nix-support.yml	Introduces the dependabot-nix-support feature flag for conditional rendering via {% ifversion %}.
content/code-security/reference/supply-chain-security/dependabot-options-reference.md	Adds a version-gated “Nix flakes” row under package-ecosystem and includes minor formatting fixes in nearby sections.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 0 new

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