Refresh devcontainer and GitHub Codespaces docs

[IEvangelist]

Summary

Refresh the Dev Containers and GitHub Codespaces getting-started docs to match the current Aspire CLI-first experience.

What changed

• update both guides to point to dotnet/aspire-devcontainer
• remove the old Aspire 9.1 rollout framing
• update examples to use the current .NET 10 devcontainer image
• switch setup guidance from manual template installation to Aspire CLI installation via the bundled installer
• use aspire new aspire-starter --name HelloAspire in the walkthroughs
• remove the remaining explicit dotnet new install Aspire.ProjectTemplates wording from these two pages only

Validation

• previously verified the updated pages locally with pnpm dev
• per request, did not run pnpm build again here; CI can validate the final change set

Notes

• local browser checks showed the updated content rendered with the expected commands and links
• existing local /_image dev-server behavior on screenshot-heavy pages is unchanged by this doc edit and should be handled separately if needed

[Copilot]

Pull request overview

Refreshes the “Dev Containers” and “GitHub Codespaces” getting-started guides to reflect the current Aspire CLI-first flow and the updated dotnet/aspire-devcontainer template.

Changes:

• Update both guides to reference https://github.com/dotnet/aspire-devcontainer (and the corresponding GitHub “new from template” URL).
• Update walkthrough commands to use aspire new aspire-starter --name HelloAspire and remove remaining dotnet new install Aspire.ProjectTemplates wording on these pages.
• Update devcontainer examples to the .NET 10 devcontainer image and install the Aspire CLI via https://aspire.dev/install.sh.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/frontend/src/content/docs/get-started/github-codespaces.mdx	Updates Codespaces guidance to the new template repo and Aspire CLI install/usage, plus adds a current devcontainer.json example.
src/frontend/src/content/docs/get-started/dev-containers.mdx	Updates Dev Containers guidance to the new template repo, Aspire CLI install/usage, and modernized devcontainer.json examples.

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