Blog posts should be truncated in list view by default

[slorber]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on 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 <!-- truncate --> 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 #9657 (comment), cc @johnnyreilly

Self-service

• I'd be willing to do some initial work on this proposal myself.