Add documentation on influencing the HTML head from Blazor

[MackinnonBuck]

[EDIT by guardrex to add the metadata]

A new preview8 feature enables influencing the HTML head element from Blazor.

When rendered, the Title, Link, and Meta components add or update data in the HTML head.

Usage example:

@using Microsoft.AspNetCore.Components.Web.Extensions

<Title Value="New document title" />
<Link rel="stylesheet" href="styles.css" />
<Meta name="description" content="Influencing the head from a Blazor component" />

Here are a few notes about the feature:

• Server-side prerendering is supported.
• Only the Value parameter is valid in Title. For Meta and Link, all provided parameters will be reflected in the DOM.
• When there are multiple Title components being rendered, the title of the page will reflect that of the Title that was most recently rendered.
• Even if there are multiple Meta components being rendered with identical attributes, there will be exactly one meta tag per Meta component (in other words, two Meta components can't refer to the same meta tag). This also applies to Link components.
• Changes to parameters of existing Meta components will be reflected by its meta tag (also applies to Link).
• Disposing a Meta component removes its meta tag from the DOM (also applies to Link).

Let me know if you would like additional information.

---

Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: 81a32115-33f8-7d06-aaf0-b8ce918b5527
• Version Independent ID: 31ad18df-1904-bbdd-cfed-8d0d0267c10e
• Content: Create and use ASP.NET Core Razor components
• Content Source: aspnetcore/blazor/components/index.md
• Product: aspnet-core
• Technology: aspnetcore-blazor
• GitHub Login: @guardrex
• Microsoft Alias: riande

[guardrex]

Thanks @MackinnonBuck ... I'll get on this one soon ... probably in a day or two if nothing else pops up on the radar. I'm going to try to 🎷 JAM! 🎸 on the 5.0 Blazor content issues as they arise to make our lives a lot easier later.

Engineering PR 👉 dotnet/aspnetcore#23833

[guardrex]

@mkArtakMSFT This one looks 🔥 HOT! 🔥, so perhaps we should place it at the bottom of the Components overview topic as opposed to placing it into the Fundamentals > Additional scnearios topic. I think it will get better visibility in the overview.

Another alternative would be the Layouts topic ... but perhaps a little less visible if placed there, too.

[mkArtakMSFT]

@guardrex, let's go with Fundamentals > Additional scenarios for now.