Adding devcontainer for development ease

[RohanRusta21]

What this PR does:

Adds a Dev Container so contributors can get a working environment without
installing the toolchain by hand.

• .devcontainer/devcontainer.json — one file, based on
  mcr.microsoft.com/devcontainers/go:1.26-trixie, which ships the Go toolchain,
  the Go tools (gopls, dlv) and make/gcc that go test -race needs.
• Features: Node 24.10.0 (which also installs yarn) and Docker-in-Docker.
• onCreateCommand installs kind v0.32.0, kubectl and Helm 3.17.3, and pre-fetches
  dependencies (go mod download, make setup-envtest, make update/web-deps) in
  parallel, so make build, make test and make check work on first open.
• Forwards 8080 (Control Plane port-forward) and 9090 (web dev server).
• Recommends the Go, ESLint and Prettier extensions.
• CONTRIBUTING.md: replaces the "Online one-click setup for contributing"
  placeholder with usage instructions for Codespaces and VS Code.

Versions are pinned to CI (build.yaml, lint.yaml) and web/package.json, with
comments naming the source file so they get bumped together.

Notes:

• Docker-in-Docker rather than mounting the host socket, because make lint/go and
  make gen/code bind-mount ${PWD}, which only resolves for a daemon running
  inside the container.
• "moby": false is required: Moby packages are not published for Debian trixie, so
  the feature installs Docker CE instead.
• The image tracks Go 1.26 (currently 1.26.5) rather than CI's exact 1.26.2. It
  satisfies go.mod's minimum, but it is not an exact pin.
• Hugo is not included, so previewing the docs site still follows docs/README.md.
• On arm64, make gen/code and make run/pipecd need
  docker run --privileged --rm tonistiigi/binfmt --install amd64 first, since both
  are amd64-only in this repo today. Everything else runs natively.

Why we need it:

Setting up a local environment today means installing eight tools whose versions are
spread across five files (go.mod, web/package.json, .github/workflows/build.yaml,
.github/workflows/lint.yaml, docs/README.md), and several requirements are not
documented anywhere:

• make test/go passes -race, so it needs a C compiler; the failure does not say so.
• make test/go also needs KUBEBUILDER_ASSETS, i.e. make setup-envtest must run first.
• make lint/go and make gen/code need Docker, and break on setups where ${PWD}
  is not visible to the daemon.
• The web app needs yarn 1.x to match web/yarn.lock.

It also helps Windows contributors, who currently need WSL — docs/README.md notes
that make and grep are not available in PowerShell. And running
make up/local-cluster inside Docker-in-Docker keeps the kind cluster, the
kind-registry container and the pipecd-data volume off the contributor's own Docker
and kubeconfig.

Finally, CONTRIBUTING.md has been advertising this as pending — "We are preparing
Gitpod and Codespace to facilitate the setup process for contributing." This implements
the Codespaces and VS Code half of that.

Which issue(s) this PR fixes:

Fixes #6581

Does this PR introduce a user-facing change?: No

• How are users affected by this change: Not affected. This changes only the
  contributor environment; no PipeCD component, API, configuration or chart is touched.
• Is this breaking change: No
• How to migrate (if breaking change): N/A

[github-actions]

👋 Hi @RohanRusta21, welcome to PipeCD and thanks for opening your first pull request!

We’re really happy to have you here

Before your PR gets merged, please check a few important things below.

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Helpful resources

• Contributing guide: https://github.com/pipe-cd/pipecd/blob/master/CONTRIBUTING.md
• Development setup: https://github.com/pipe-cd/pipecd/blob/master/CONTRIBUTING.md#development

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DCO Sign-off

All commits must include a Signed-off-by line to comply with the Developer Certificate of Origin (DCO).

In case you forget to sign-off your commit(s), follow these steps:

For the last commit:

git commit --amend --signoff
git push --force-with-lease

For multiple commits:

git rebase --signoff origin/master
git push --force-with-lease

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Run checks locally

Before pushing updates, please run:

make check

This runs the same checks as CI and helps catch issues early.

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💬 Need help?

If anything is unclear, feel free to ask in this PR or join us on the CNCF Slack in the #pipecd channel.
You can get your Slack invite from: https://communityinviter.com/apps/cloud-native/cncf

Thanks for contributing to PipeCD! ❤️