Document template parts used by the Classic Template block

[Aljullu]

Changes proposed in this Pull Request:

This PR updates the documentation of the Classic Template block so:

1. We unify two files where the documentation was split.
2. We mention the template parts used by the Classic Template block, closing Document which templates parts are used by the Classic Template Placeholder block #47219. That documentation was requested by 3rd-party devs.
3. We also mention PHP hooks which are run in the Classic Template block.
4. We mention that we discourage both of those extensibility methods if they are not accompanied by specific blocks serving the same function in the blockified templates.

Closes #47219.

How to test the changes in this Pull Request:

1. Read the documentation and make sure it's correct.

Changelog entry

• Automatically create a changelog entry from the details below.

• This Pull Request does not require a changelog entry. (Comment required below)

Changelog Entry Details

Significance

• Patch
• Minor
• Major

Type

• Fix - Fixes an existing bug
• Add - Adds functionality
• Update - Update existing functionality
• Dev - Development related task
• Tweak - A minor adjustment to the codebase
• Performance - Address performance issues
• Enhancement - Improvement to existing functionality

Message

Changelog Entry Comment

Comment

[github-actions]

Test using WordPress Playground

The changes in this pull request can be previewed and tested using a WordPress Playground instance.
WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Test this pull request with WordPress Playground.

Note that this URL is valid for 30 days from when this comment was last updated. You can update it by closing/reopening the PR or pushing a new commit.

[github-actions]

Hi @WunderBart, @imanish003,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[imanish003]

Hey @Aljullu,

Thanks for improving the documentation of the Classic Template block. I've reviewed the changes and they look good to me. I've left a few minor suggestions. I am pre-approving this PR so you can merge it once the suggestions are addressed 🚀