postgraphile website: links fixes

[jemgillam]

Description

Draft PR
I've opened this early so I can keep note on which folders link to which versions of the documentation.

Fixing links across the postgraphile.org site.

• Links within /postgraphile ("Draft version") link to build.graphile.org/graphile-build/5/
• Links within /versioned_docs/version-4 link to build.graphile.org/graphile-build/4/
• Links within /versioned_docs/version-5 link to build.graphile.org/graphile-build/5/

And this is reflected in /versioned_docs/version-latest ("Current version") which links to build.graphile.org/graphile-build/4/ as it is merely a symlink to /versioned_docs/version-4

The exception is linking to the Look Ahead feature, as that doesn't exist in Version 5 documentation it always links to build.graphile.org/graphile-build/4/look-ahead

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Latest commit: b3470ed

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