docs(misc): give nx cloud seekers a tagged path from the intro page (#36842)

## Current Behavior

The intro page has no links to the Nx Cloud app, and the cache and
ci-features pages have no browser CTA. The docs header button says "Get
started" and is tagged `utm_source=nx-dev`, so docs traffic is
indistinguishable from the marketing site.

## Expected Behavior

The intro page describes what Nx Cloud does and links the app from the
feature table and from "Where to go from here". The cache and
ci-features pages get the browser CTA used on the other caching pages.
The header button reads "Try Nx Cloud for Free", matching the homepage.
Every docs app link uses `utm_source=nx-docs&utm_medium=website` with a
per-placement `utm_campaign`.

## Preview

-
https://deploy-preview-36842--nx-docs.netlify.app/docs/getting-started/intro
-
https://deploy-preview-36842--nx-docs.netlify.app/docs/features/ci-features
-
https://deploy-preview-36842--nx-docs.netlify.app/docs/features/cache-task-results
-
https://deploy-preview-36842--nx-docs.netlify.app/docs/getting-started/nx-cloud
(UTM retag only)
-
https://deploy-preview-36842--nx-docs.netlify.app/docs/getting-started/setup-ci
(UTM retag only)

The header button change is site chrome, visible on every page.

## Related Issue(s)

DOC-662