feat(blog-plugin): limit option for blog feedOptions

[johnnyreilly]

Breaking change

With Docusaurus v3, the blog feeds are now limited to the last 20 items by default

This permits ensuring the feeds keep a reasonable max size and ensure by default feed reader apps and SaaS are able to read that feed successfully.

If you want to revert to the old Docusaurus v2unlimited behavior, use limit: false

Note: if you use the createFeedItems() hook, the items you receive have already been limited (by default to 20). You might need to turn off this behavior for advanced createFeedItems() customizations involving access to the whole list of blog posts.

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes Blog feed: add a first-class API to limit size of the RSS / Atom feeds #9179) and the maintainers have approved on my working plan.

Motivation

To make limiting the size of a blog feed straightforward and the default behaviour.

Test Plan

Test links

Deploy preview:

• https://deploy-preview-9189--docusaurus-2.netlify.app/docs/api/plugins/@docusaurus/plugin-content-blog/
• https://deploy-preview-9189--docusaurus-2.netlify.app/docs/blog/#feed

Related issues/PRs

#8378

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URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
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[slorber]

LGTM thanks 👍

My only concern is that I hope it's clear that the limit behavior is applied before the createFeedItems, because now we somehow have 2 ways to limit the feed. I guess if this becomes a problem we can recommend users of the createFeedItems to turn off the default limit so that they can fully customize the final feed through createFeedItems