Subtheme: Update documentation around subtheming

So the basic premise that we wanted to add body classes with certain information to it, is already covered. By default we get classes:
"node-2 node-type--sector-page" as per https://git.drupalcode.org/project/sector_theme/-/blob/1.0.x/includes/ht...

Update: Steps reproduce Sub theme errors.

When I run:

php core/scripts/drupal generate-theme --starterkit sector_starterkit NewSubTheme --path themes/custom

I get:

Directory themes/custom/NewSubTheme could not be created - so this needs to be done manually

So I created the custom folder manually and then the php script works. Updating DOCS. (or we could add a .gitkeep file in the folder to make it remain even when empty?)

Then, I install the theme, from the UI as it didn't want to run from
`drush en SUBTHEME_NAME -y; drush config-set system.theme default SUBTHEME_NAME -y`

Then I got this WSOD:

Drupal\sdc\Exception\InvalidComponentException: [id] Does not match the regex pattern ^[a-z]([a-zA-Z0-9_-]*[a-zA-Z0-9])*:[a-z]([a-zA-Z0-9_-]*[a-zA-Z0-9])*$ in Drupal\sdc\Component\ComponentValidator->validateDefinition() (line 119 of core/modules/sdc/src/Component/ComponentValidator.php).

I then removed this theme and recreated a new theme, using a lower case name, and then I get no errors. So sub theme documentation perhaps should refer to: SUBTHEME_NAME in lower case?

stewest → credited gareth.poole → .

The latest updates, fixing the sub theme generation solves the issues mentioned above.