Notifications: audit and tune notification body and subject copy

[humitos]

In #10922 I migrated all the messages exposed to users to the new notification system, but I haven't change them. There are plenty of them that are not super clear and can be improved to communicate the message in a simpler way.

Besides, since there is a new header copy now, we should decide how they are going to be shown to users and make them work in that context as well.

• Example: New notification system: implementation #10922 (comment)
• Missing header: New notification system: implementation #10922 (comment)
• Default header: New notification system: implementation #10922 (comment)

[agjohnson]

I'm happy to help with some of the copy and grammar fixes here. There will be a fair amount to go through, I wasn't thorough in my review.

[humitos]

These are all the issues that I quickly found where users were confused about the error displayed:

• YAML config: errors are not clear enough for users #10502
• Better error when using conda without an environment file #9008
• Config: check for conda constraints and raise a build error #6569
• Cause of build error not indicated if there is a typo in the path to the documentation #7707
• Catch enconding errors when reading the .readthedocs.yaml file #8482
• Wrong sphinx.configuration path causes not correct error message #8352
• Build config: validate that elements of extra_requirements are strings #9458
• Build: use Sphinx configuration file explicitly #9713
• Submodules: user message when there are problems cloning them #9365

[humitos]

Some of the issues linked here will require modifications in the Config validation object, not only improves in the error messages. We can track those as a different set of work.