docs: clarify dev container scope

[zhouzhou626]

Fixes

Fixes #2937 by @sarayourfriend

Description

Adds a note near the top of the general setup guide explaining that VS Code's .devcontainer/devcontainer.json prompt should not be used for local setup. The note clarifies that the dev container configuration is intended and tested for GitHub Codespaces, specifically for frontend and documentation contributions, while local development should use ov directly.

Testing Instructions

Documentation-only change. I verified the changed file with:

git diff --check
rg -n "dev container configuration|GitHub Codespaces|VS Code detects" documentation/general/general_setup.md

I did not run the full documentation build because this local environment does not have the project's pdm or just tooling installed.

Checklist

• My pull request has a descriptive title (not a vague title likeUpdate index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• [N/A] I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• [N/A] I ran the DAG documentation generator (ov just catalog/generate-docs for catalog
  PRs) or the media properties generator (ov just catalog/generate-docs media-props
  for the catalog or ov just api/generate-docs for the API) where applicable.

Developer Certificate of Origin

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