Add contentDirName option

[jajaperson]

Why:

Closes: #4782

What's being changed (if available, include any code snippets, screenshots, or gifs):

This adds a contentDirName option to the Nextra config, and the CONTENT_DIR is now set via

function getContentDirectory(dirName: string) {
  // Next.js gives priority to `app` over `src/app`, we do the same for the `content` directory
  const [contentDir = dirName] = fg.sync([`{src/,}${dirName}`], {
    onlyDirectories: true,
  })
  return contentDir
}

Check off the following:

• I have reviewed my changes in staging, available via the View
  deployment link in this PR's timeline (this link will be available after
  opening the PR).

[changeset-bot]

🦋 Changeset detected

Latest commit: 63b7849

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
nextra	Major
nextra-theme-blog	Major
nextra-theme-docs	Major

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Preview	Comments	Updated (UTC)
nextra-v2	Ready	Preview	Comment	Nov 8, 2025 9:31am

[vercel]

@jajaperson is attempting to deploy a commit to the Vercel Team on Vercel.

A member of the Team first needs to authorize it.

[jajaperson]

Hang on, the doc's aren't quite right let me fix that.