chore(ci): add node 26 to workflows matrices

[slorber]

Motivation

We'll likely release Docusaurus v4 at a time where v26 is, or close to be, the new LTS, so let's ensure our CI tests against it.

I've only noticed this warning for now. It seems related to a Mermaid dependency and doesn't block the build.

[PERF] Build > en > Creating rspack bundler configs - 61.59 ms - (Heap 178mb -> 173mb / Total 262mb)
● Client ██████████████████████████████████████████████████ (100%) emitting after emit                                                                                                                             
● Server ██████████████████████████████████████████████████ (100%) emitting after emit

[WARNING] {"message":"  ⚠ Critical dependency: require function is used in a way in which dependencies cannot be statically extracted\n   ╭─[3:24]\n 1 │ (function (factory) {\n 2 │     if (typeof module === \"object\" && typeof module.exports === \"object\") {\n 3 │         var v = factory(require, exports);\n   ·                         ───────\n 4 │         if (v !== undefined) module.exports = v;\n 5 │     }\n   ╰────\n","moduleIdentifier":"/projects/docusaurus/node_modules/vscode-languageserver-types/lib/umd/main.js","moduleName":"../node_modules/vscode-languageserver-types/lib/umd/main.js","loc":"3:25-31","compilerPath":"server"}

Test Plan

CI

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

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Build	Status	Details	Updated (UTC)
default (Inspect)	✅ No changes detected	-	May 7, 2026, 3:10 PM