Improve grammar and writing in Pipeline compilations page (#10257)

* Improve grammar and writing in Pipeline compilations page

- Mention example of off-screen rendering using ColorRect to hide
  the root viewport's 3D rendering or a SubViewport node.
- Split into more paragraphs to improve readability.
- Link to class reference pages when relevant.
- Wrap source file to 80 characters like other pages.

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Co-authored-by: Max Hilbrunner <[email protected]>
Co-authored-by: tetrapod <[email protected]>