Add Virtual Scroll component topic

[viktorkombov]

Closes #831
Closes #832

Adds a new Virtual Scroll topic, built to the igniteui-doc-topics house style.

Changes

• New docs/xplat/src/content/en/components/layouts/virtual-scroll.mdx, shared by Angular and Web Components and excluded for React and Blazor.
• TOC entries in docs/xplat/src/content/en/toc.json and docs/angular/src/content/en/components/toc.json.
• for-of.mdx: adds a note that recommends Virtual Scroll for new single-axis lists and links to the igxForOf migration table. It does not deprecate the directive.

Dependencies

Repo	Link	Status
igniteui-angular	rkaraivanov/virtual-scroll-component #17281	Not released
igniteui-angular-samples	IgniteUI/igniteui-angular-samples#4029	Samples
igniteui-wc-examples	IgniteUI/igniteui-wc-examples#1268	Samples

Before merge

• Replace ‹VERIFY› with the first igniteui-angular release that ships the component.
• Add the Anatomy and Do/Don't images; the topic has TODO markers for them.
• Regenerate the API link registry. VirtualScroll links render as plain code until then.

Checklist:

• check topic's TOC/menu and paragraph headings
• Include TOC topic labels in the topic content when it has a valuable update, is new, or is considered preview / beta
• link to other topics using ./page.mdx or ../relative/path.mdx (.mdx extension required)
• at the References section at the end of the topic add links to topics, samples, etc
• reference API documentation instead of adding a section with API

• use valid component names - [Data] Grid, IgxSelectComponent, <igx-combo>
• use spell checker tool (VS Code, Grammarly, Microsoft Editor)
• add inline code blocks for the names of classes / tags / properties
• add language descriptor for the code blocks
• check if links function by running the check-api/mdx npm commands
• check if sample is working and fully visible in the topic
• check if sample is working and fully visible in the StackBlitz
• check if code blocks match the code in StackBlitz demo
• add or update meaningful llms.description metadata and run npm run check:llms-metadata

• follow SEO guidelines to fill topic's metadata (SEO guidelines)

• do not resolve requested changes (leave that to the reviewer)