docs: document CSS custom property namespacing in the styling guide - #70835

…70835)

`provideCssVarNamespacing` and `CssVarNamespacer` landed in v22.1 (#68846)
with no guide coverage, so the generated API reference was the only
description of them. That reference cannot explain where the feature stops:
Angular rewrites the custom property names in the styles it compiles, but
static `style` attributes, object style bindings, `Renderer2.setStyle`,
global stylesheets, and library components that read a global theme all keep
the original name, and nothing reports the mismatch at build time or at
runtime.

Document the feature in the component styling guide: what Angular rewrites,
the surfaces that keep the name you write, how to opt a name out with
`--global--`, and how to resolve a namespaced name from TypeScript.

Fixes #70834

PR Close #70835

…PIs (#70835)

Point the TSDoc of `provideCssVarNamespacing` and `CssVarNamespacer` at the
guide section that describes them, so the generated API pages lead back to
it, and name the component file in the guide's example.

PR Close #70835

…PIs (angular#70835)

Point the TSDoc of `provideCssVarNamespacing` and `CssVarNamespacer` at the
guide section that describes them, so the generated API pages lead back to
it, and name the component file in the guide's example.

PR Close angular#70835