remove mention of carpetcode.org from ET website

[Roland Haas]

https://carpetcode.org has been retired. Since Carpet’s website has not been updated in many years, no alternative website will be provided and references to it should be removed from all ET documentation and websites. Possibly to be replaced by https://bitbucket.org/eschnett/carpet as the URL to the Carpet code repository where appropriate.

[Roland Haas]

• updated ET website in git hash 1da4d4c "replace carpetcode.org by bitbucket.org/eschnett/carpet.git" of www
• updated GitHub cactuscode.org in git hash 6a354d7 "replace carpetcode.org with bitbucket.org/eschnett/carpet.git" of cactuscode.org
• updated einsteintoolkit.bib in git hash 5532e3b "einsteintoolkit.bib: replace careptcode.org by bitbucket repo" of manifest
• updated Carpet’s README and docs in git hash 7931af13 "doc: remove most references to carpetcode.org" of carpet
• carpetcode.org is not mentioned in the ET wiki at https://docs.einsteintoolkit.org

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