docs: deprecate create-eth extensions

[rin-st]

What

Mark create-eth extensions as deprecated across the docs. No code changes — extensions still ship and work for now; this is docs-only, with removal to follow in a later phase.

Changes

• CONTRIBUTING.md — drop the "create a PR to the example extension branch" contributor guideline.
• contributors/THIRD-PARTY-EXTENSION.md — add a deprecation banner ([!WARNING]); guide kept for existing extension authors.
• contributors/TEMPLATING.md / TEMPLATE-FILES.md — add deprecation note ([!NOTE]); templating mechanics kept, since create-eth still uses the template/args system internally to assemble instances.
• contributors/DEVELOPER-GUIDE.md — reframe the templating intro away from extension-authoring; mark the args-sync checklist item (optional, deprecated).

All deprecation notes point users to skills as the replacement: https://docs.scaffoldeth.io/build-with-ai

Removed extensions from .github/workflows/lint-instances.yml:

• Matrix reduced from 6 → 3 configs: [hardhat, foundry, no_solidity_framework] (dropped the three _with_extension variants).
• Removed the 3 "Install application … with extension" steps.
• Simplified the 6 if: conditionals that referenced _with_extension.

Notes

• The user-facing README note (skills instead of extensions) is not in this PR — it'll land in scaffold-eth-2 and reach create-eth via backmerge.

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