Create the LLMs.txt automatically and on a schedule.

[leonidasmi]

Context

Summary

This PR can be summarized in the following changelog entry:

• Creates the LLMs.txt automatically and on a schedule.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Switch to this PR and deactivate and activate the Yoast SEO plugin (or, if you're QA, install and activate this RC in a "fresh" site (as in, a site that never had this code before) ).
• Using WP Crontrol, confirm that no scheduled wpseo_llms_txt_population action is created. And no llms.txt file is created.
• Toggle the setting on and confirm that there's the scheduled action is set for the next week and that the llms.txt file is also instantly created.
• Remove the cron via the WP Crontrol plugin and file from the file system.
• Deactivate and activate the Yoast SEO plugin.
• Using WP Crontrol, confirm that a weekly wpseo_llms_txt_population scheduled action was created, that's about to run in the next 5 minutes
• Wait 5 minutes and confirm that a (empty if Add content in the llms.txt file #22257 hasnt been merged yet) /llms.txt file has been created in the root of the server and that's accessible in example.com/llms.txt
• Also confirm that the scheduled action is now set to run in a week.
• Manually delete the llms.txt file and run the scheduled action. Confirm that the file was regenerated.
• Toggle the setting off and confirm that there is no scheduled action anymore and that the llms.txt file was deleted
• Do the same tests but for the first step upgrade to this RC instead of installing it in a new site. Things should still behave the same.

Upgrade routines (no longer relevant after #22281)

• In a fresh site that didnt have this code before have Yoast SEO activate and checkout this branch. Run the upgrade routine
  • Or if you're QA, install the released version and upgrade to this RC.
• Using WP Crontrol, confirm that there's no scheduled wpseo_llms_txt_population action.
• Toggle the setting on and confirm that the wpseo_llms_txt_population got created.
• Remove the cron via the WP Crontrol plugin and file from the file system.
• Re-run the upgrade routine and confirm that the wpseo_llms_txt_population was created again, to be ran in 5 minutes.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes https://github.com/Yoast/reserved-tasks/issues/537

[leonidasmi]

CR: 🏗️

[github-actions]

A merge conflict has been detected for the proposed code changes in this PR. Please resolve the conflict by either rebasing the PR or merging in changes from the base branch.

[leonidasmi]

CR + Acceptance is ✅