chore: add workspaceConcurrency to settings

[o-az]

Adds the workspaceConcurrency setting to the settings documentation.

This setting was already available via the CLI (--workspace-concurrency) but was missing from the settings reference page.

Based on the source code I understand that this is a valid setting in pnpm-workspace.yaml:

https://github.com/pnpm/pnpm/blob/main/config/config/src/types.ts#L131
https://github.com/pnpm/pnpm/blob/main/config/config/src/index.ts#L608

Also verified locally by doing the following:

• setting workspaceConcurrency to 1 in pnpm-workspace.yaml,
• running

   pnpm --recursive exec -- sh -c 'echo "$(date +%H:%M:%S) START $PNPM_PACKAGE_NAME" \
     && sleep 3 \
     && echo "$(date +%H:%M:%S) END $PNPM_PACKAGE_NAME"'

• changing workspaceConcurrency to -1 in pnpm-workspace.yaml,
• running

   pnpm --recursive exec -- sh -c 'echo "$(date +%H:%M:%S) START $PNPM_PACKAGE_NAME" \
     && sleep 3 \
     && echo "$(date +%H:%M:%S) END $PNPM_PACKAGE_NAME"'

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