Changes indexables page copy

[thijsoo]

Context

• We want to explain everything that happens on the indexables page

Summary

This PR can be summarized in the following changelog entry:

• Updates copy on the indexables page

Relevant technical choices:

• None.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Open https://basic.wordpress.test/wp-admin/admin.php?page=wpseo_dashboard and make sure you have no focus keyphrases in your posts or set The minimum threshold for the amount of analyzed posts to 100% in the test helper.
• Compare the copy of the Posts and pages without a focus keyphrase Panel with https://www.sketch.com/s/9bdad4d5-3c03-4b39-8e7e-345935c75f6c/prototype/a/7A49BE9A-C79E-4FDE-A6FD-4B32151D5114
• Add keyphrases to your posts or set the threshold to 1%.
• Compare the copy in the panels to the copy in https://www.sketch.com/s/9bdad4d5-3c03-4b39-8e7e-345935c75f6c/prototype/a/054D5645-F179-4936-BD18-904E684DAD90

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 (Block/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.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

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.

Quality assurance

• I have tested this code to the best of my abilities
• 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.

Fixes #
PC-562