doc: normalize markdown for nixos-render-docs

pandoc recognizes `::: note` admonitions, nixos-render-docs only
recognizes `::: {.note}`. surprisingly pandoc also emits the correct
docbook tags for `[](#xref)`s, so we can use that too.

doc/builders/testers.chapter.md

···
 # Testers {#chap-testers}
-This chapter describes several testing builders which are available in the <literal>testers</literal> namespace.
+This chapter describes several testing builders which are available in the `testers` namespace.
 
 ## `hasPkgConfigModule` {#tester-hasPkgConfigModule}
 

doc/stdenv/meta.chapter.md

···
 * we can run `passthru.tests` independently
 * `installCheckPhase` adds overhead to each build
 
-For more on how to write and run package tests, see <xref linkend="sec-package-tests"/>.
+For more on how to write and run package tests, see [](#sec-package-tests).
 
 #### NixOS tests {#var-meta-tests-nixos}
 

doc/stdenv/stdenv.chapter.md

···
 
 The installCheck phase checks whether the package was installed correctly by running its test suite against the installed directories. The default `installCheck` calls `make installcheck`.
 
-It is often better to add tests that are not part of the source distribution to `passthru.tests` (see <xref linkend="var-meta-tests"/>). This avoids adding overhead to every build and enables us to run them independently.
+It is often better to add tests that are not part of the source distribution to `passthru.tests` (see
+[](#var-meta-tests)). This avoids adding overhead to every build and enables us to run them independently.
 
 #### Variables controlling the installCheck phase {#variables-controlling-the-installcheck-phase}
 
···
 
 This setup hook patches installed scripts to add Nix store paths to their shebang interpreter as found in the build environment. The [shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) line tells a Unix-like operating system which interpreter to use to execute the script's contents.
 
-::: note
+::: {.note}
 The [generic builder][generic-builder] populates `PATH` from inputs of the derivation.
 :::
 
···
 
 Interpreter paths that point to a valid Nix store location are not changed.
 
-::: note
+::: {.note}
 A script file must be marked as executable, otherwise it will not be
 considered.
 :::