docs: give the last step of the prevnext chain its way back

[lens0021]

Closes the one gap left in #455's evidence. Most of what that issue reported was fixed hours earlier by #452, which wired Commands, Development and Writing an extension into the chain; the issue has been corrected to say so. What #452 could not reach is the end of the chain.

The gap

Writing an extension's next link points at Licenses, and Licenses carries no navigation at all. A reader who follows that link arrives at the last page of the documentation with no way back and nothing telling them they have reached the end.

It could not have one. The template read a single argument as the next step, so the two ends of the chain were not symmetrical:

	before
first page	`{{prevnext	Installation}}`
last page	—	nothing after it — not expressible

The change

The first argument is the previous step and the second is the next, each side left empty where the chain ends:

{{prevnext|previous step|next step}}
{{prevnext||next step}}
{{prevnext|previous step|}}

That drops the template's {{{2|{{{1}}}}}} fallback — the one that made a lone argument mean "next" — so both edges now read the same way, and the template is shorter than it was.

Rendering changes for exactly one page. index and Why wikitext move to the empty-first-argument form and come out byte-identical; Licenses gains the row it was missing.

What it looks like now

The chain walks the sidebar's 19 entries in the sidebar's order and stops where the sidebar does:

index → Why wikitext → Installation → Getting Started → Pages → Editing sidebar → Images
      → Skins → Extensions → JavaScript → Searching → Translating → Deploying
      → Troubleshooting → Configuration → Standalone binary → Commands → Development
      → Writing an extension → Licenses ∎

Every sidebar entry has a prevnext, every back-link points at the page that links forward to it, and index remains the head that the sidebar does not list.

Not in this change

The order is still written twice, once here and once as MediaWiki:Sidebar. This fixes the drift, not its cause — #455 holds the proposal for the cause, including the cheaper half of it: a check that the chain and the sidebar agree, which is what would have caught both this gap and the one #452 closed.

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Generated by Claude Code

[lens0021]

Closed as a duplicate of #456, which was opened 34 seconds earlier and fixes the same gap: Licenses had no navigation because the template could not spell "a previous step and no next".

The two differ only in how they spell it, and #456's way is the safer one. It adds a named prev= and leaves the positional forms alone, so every existing call keeps its meaning. This one made the first argument always the previous step and the second always the next, which reads more uniformly and shortens the template, but it silently redefines {{prevnext|X}} from "next is X" to "previous is X" — fine for the 20 calls updated in the same commit, and a trap for the 21st written from memory or copied out of an older revision.

One thing from here worth keeping if it is not already covered: the chain and the sidebar were checked entry by entry, and with Licenses fixed they agree exactly — 19 sidebar entries, all with a prevnext, in the sidebar's order, every back-link pointing at the page that links forward to it, and index as the head the sidebar does not list. That agreement is what #455 is about keeping true.

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Generated by Claude Code