docs: Note for CI missing docs that pnpm isn't supported

[bmulholland]

See also #5880

[netlify]

👷 Deploy request for cypress-docs pending review.

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[cypress-app-bot]

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[jennifer-shehane]

@MikeMcC399 thoughts?

[MikeMcC399]

@jennifer-shehane

This PR was proposing to make changes to https://docs.cypress.io/guides/continuous-integration/introduction#Caching.
The section does not cover specifics for the Cypress JavaScript GitHub Action (cypress-io/github-action) and is intended for general use.

• There was never any restriction on caching across builds with pnpm without cypress-io/github-action.

• The temporary restriction on manually caching the pnpm store across builds was lifted with the resolution of Provide pnpm workflow caching without action enhancement github-action#1227. The action cypress-io/github-action in any case caches the Cypress binary and the npm cache across builds and this cannot be disabled.

I suggest to close this PR.