feat: WP-197 use @layer's / help migrate Frontera

[wesleyboar]

Overview

Use @layer for Core-CMS stylesheets loaded when TACC_CORE_STYLES_VERSION=0.

Frontera migration — https://github.com/TACC/Core-CMS-Custom/pull/293 — uses @layers, but then Frontera styles yield to Core-CMS styles. They yield to Core-CMS because Core-CMS — with `TACC_CORE_STYLES_VERSION=0` — has all its styles in an anonymous layer. (CSS in an anonymous layer beats CSS in a named layer.)

Related

• WP-197
• required by https://github.com/TACC/Core-CMS-Custom/pull/293

Changes

1. updated link to CSS organization doc
2. refactored site assets to load in @layer's
3. refactored delayed site assets to mirror core's ^{(noop cuz the latter has been just a cleaned up version of the former)}
4. refactored blog assets to load in @layer ^{(only to match TACC_CORE_STYLES_VERSION=2 CMS use of @layer's)}

Testing / UI

1. 2. 3. Tested via https://github.com/TACC/Core-CMS/pull/856.

4. Untested. Only ECEP could be affected, but unlikely. ^{Unlikely, because ECEP CSS is so simple.}