Add note on CSP to Compression Dictionary Transport (#42487)

* Update Compression Dictionary Transport with a note on CSP implementation

* Moved CSP notice to Restrictions

* Add note on CSPs on link rel=compression-dictionary page

* Increased verbosity

* Corrected pluralisation

* Reverted to singular language

* Aligned paragraph with other document

* Update files/en-us/web/http/guides/compression_dictionary_transport/index.md

* Update files/en-us/web/http/guides/compression_dictionary_transport/index.md

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

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Co-authored-by: Hamish Willee <hamishwillee@gmail.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>