Create extension page

[JeroenDeDauw]

At https://professional.wiki/en/extension/persistent-page-identifiers

Similar to https://professional.wiki/en/extension/page-approvals

We could put the installation instructions on the page instead of GitHub README. Would be nice to also link the Release notes under External links.

Once published: link from

• https://github.com/ProfessionalWiki/PersistentPageIdentifiers (repo metadata URL)
• https://github.com/ProfessionalWiki/PersistentPageIdentifiers/blob/master/extension.json
• https://github.com/ProfessionalWiki/PersistentPageIdentifiers/blob/master/composer.json
• The MediaWiki.org page