sphinx/conf.py: Add redirect for devel/swh-model/persistent-identifiers

That file was renamed to devel/swh-model/swhids in swh-model!341 so ensure to not break previous URL by redirecting to the new one.

mentioned in merge request swh-model!341

Jenkins job DDOC/builds #511 succeeded .
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added 7 commits

• 255d0a08...c8d28696 - 6 commits from branch swh/devel:master
• ea5dd517 - sphinx/conf.py: Add redirect for devel/swh-model/persistent-identifiers

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^ Rebase

Jenkins job DDOC/builds #532 succeeded .
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added 93 commits

• ea5dd517...ce380c2d - 92 commits from branch swh/devel:master
• 7afaa6da - sphinx/conf.py: Add redirect for devel/swh-model/persistent-identifiers

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Jenkins job DDOC/builds #808 succeeded in 17 min.
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