doc: make theme consistent across api and other docs

Since website based on 2 different repos, there was an inconsistency
in theme selection, so we had 2 independant theme props.
Now only one stored in local storage is a single source of truth

BUILDING.md

@@ -462,6 +462,13 @@ make docopen
 This will open a file URL to a one-page version of all the browsable HTML
 documents using the default browser.
 
+
+```bash
+make docclean
+```
+
+This will clean previously built doc. 
+
 To test if Node.js was built correctly:
 
 ```bash

doc/api_assets/api.js

@@ -2,11 +2,11 @@
 
 {
   function setupTheme() {
-    const kCustomPreference = 'customDarkTheme';
-    const userSettings = sessionStorage.getItem(kCustomPreference);
+    const storedTheme = localStorage.getItem('theme');
     const themeToggleButton = document.getElementById('theme-toggle-btn');
 
-    if (userSettings === null && window.matchMedia) {
+    // Follow operating system theme preference
+    if (storedTheme === null && window.matchMedia) {
       const mq = window.matchMedia('(prefers-color-scheme: dark)');
 
       if ('onchange' in mq) {
@@ -28,16 +28,16 @@
       if (mq.matches) {
         document.documentElement.classList.add('dark-mode');
       }
-    } else if (userSettings === 'true') {
+    } else if (storedTheme === 'dark') {
       document.documentElement.classList.add('dark-mode');
     }
 
     if (themeToggleButton) {
       themeToggleButton.hidden = false;
       themeToggleButton.addEventListener('click', function() {
-        sessionStorage.setItem(
-          kCustomPreference,
-          document.documentElement.classList.toggle('dark-mode'),
+        localStorage.setItem(
+          'theme',
+          document.documentElement.classList.toggle('dark-mode') ? 'dark' : 'light',
         );
       });
     }

doc/contributing/collaborator-guide.md

@@ -532,7 +532,7 @@ The TSC serves as the final arbiter where required.
    [build](https://github.com/nodejs/build/issues) repositories, open new
    issues. Run a new CI any time someone pushes new code to the pull request.
 4. Check that the commit message adheres to [commit message guidelines][].
-5. Add all necessary [metadata](#metadata) to commit messages before landing. If
+5. Add all necessary [metadata](git-node-metadata) to commit messages before landing. If
    you are unsure exactly how to format the commit messages, use the commit log
    as a reference. See [this commit][commit-example] as an example.