Blog posts should be truncated in list view by default

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#issues).

### Motivation

To prevent SEO / duplicate content issues, blog posts should be truncated in list view (paginated lists) by default.

Currently we only truncate posts having the truncate marker `` or `{/* truncate */}`, but many users don't add or know that marker exist, leading to possible SEO problems.

In Docusaurus v4, let's do a breaking change and make sure we always truncate posts by default, with an option to opt-out to the old v3 behavior (having the ability to not truncate).

We could use the blog post description/excerp as the default truncated version (unless user provides an explicit truncate marker).

Related to https://github.com/facebook/docusaurus/issues/9657#issuecomment-1875092809, cc @johnnyreilly

### Self-service

- [X] I'd be willing to do some initial work on this proposal myself.