Add WP-Cron limitations and best practices page

[ekamran]

Summary

Adds a new page to the Cron chapter covering WP-Cron's practical limitations and how to work around them.

Fixes #73.

Background

I opened #73 in May listing gaps in the Cron chapter and asked whether the content should go on the overview page, on a new subpage, or be spread across existing pages. The issue did not get a response, so I have gone with the new subpage since that matches how the rest of the chapter is organised. Happy to restructure if the team prefers another shape.

Changes

• Adds cron/limitations-and-best-practices/index.md.
• Adds the matching entry to manifest/handbook-manifest.json at order 5, after Testing of WP-Cron.

No existing page is modified.

What it covers

• Performance on high-traffic sites, including the move of the cron spawn to the shutdown action in WordPress 6.9.
• Why callbacks should be safe to run more than once, what core's doing_cron lock does and does not guarantee.
• WP-Cron on managed hosting, where the page-load trigger is often disabled in favour of a system cron.
• Debugging callbacks with WP_DEBUG_LOG, and a table of common symptoms and causes.
• What to do when WP-Cron is not sufficient.

Notes

• Every technical claim is checked against core in src/wp-includes/cron.php, src/wp-cron.php and src/wp-includes/default-constants.php.
• No third-party services, hosts or libraries are named or linked, per the external linking policy.
• Headings use Title Case to match the other pages in this chapter, though the style guide asks for sentence case. Happy to switch if you would rather the new page follow the style guide.
• All four internal links return 200.

[ekamran]

@frantorres @davidperezgar this one follows up #73, which asked where this content should live and did not get a response. I have gone with a new subpage since that matches the rest of the Cron chapter.

It is one new file plus a single manifest entry, no existing page is touched. Happy to restructure if you would prefer it split differently.