Replace simple theme with startup theme

[GuySartorelli]

Pikachus have created https://github.com/silverstripeltd/startup-theme

silverstripe/startup-theme (this repository) was originally meant to house it.

This theme will become the new default theme for CMS 6, replacing the simple theme.

Acceptance Criteria

• The code from https://github.com/silverstripeltd/startup-theme is dumped into here
  • Just do a force push, retaining all the commit history from the private repo. We don't need the existing commit history from this repo.
• The steps for setting up a new GitHub repository are done for this repo
• The theme gets a review from a "does this do what a default theme needs to do" pov
• The theme gets a lightweight code quality review
• The readme is updated if needed (ideally include a screenshot)
• This repo is added to the supported modules docs list and supported modules repo
• Module standardiser is run against this repo
• This theme is added to installer
• simple theme is removed from installer
• Any docs referencing simple theme are updated to reference this repo instead for CMS 6 docs
• simple theme is removed from the supported modules docs list and supported modules repo for CMS 6
• Changelog explicitly calls out the change of theme and points out anything that people might need to be aware of

PRs

Kitchen sink CI run

• ENH Prepare theme for adoption as a supported module #9
• Various required changes #10
• ENH Add new theme as CMS 6 supported module supported-modules#53
• ENH Account for startup theme module-standardiser#76
• ENH Add exclusion for startup-theme which has no API to document api.silverstripe.org#130
• DEP Swap out simple theme for the new startup theme silverstripe-installer#384
• FIX Remove unnecessary theme.yml file. recipe-kitchen-sink#85
• MNT Update themes reference in .gitignore recipe-cms#94
• MNT Update themes reference in .gitignore recipe-core#104
• MNT Update theme reference in text fixture silverstripe-siteconfig#185
• MNT Update theme reference in test silverstripe-userforms#1366
• DOC Remove reference to simple theme silverstripe-behat-extension#297
• DOC Remove simple theme reference from PHPDoc comment silverstripe-framework#11594
• DOC Document new startup theme developer-docs#683

[emteknetnz]

Have merged PR, assigning back to do other steps

[emteknetnz]

@GuySartorelli I get the following issue when trying to create a sink/installer project

ERROR [User Notice]: File themes/startup/images/logo--silverstripe-cms.svg does not exist
IN GET /server-error
Line 84 in /var/www/vendor/silverstripe/framework/src/Control/SimpleResourceURLGenerator.php

[GuySartorelli]

Looks like packagist was just slow to update - I got that locally as well, but after telling packagist to update and trying again the problem went away.