docs: document go workspace setup for editor / LSP

Luke Hollindarequested to merge
lh-go-workspace-guidance-and-gitignore into master

When LabKit is opened from the repo root (may depend on editor), gopls attaches to the root module's go.mod and does not index v2/'s dependencies.

This commit adds CONTRIBUTING.md guidance on setting up a go.work file to avoid this issue.

Adds go.work and go.work.sum to .gitignore as per standard practice

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