[SPA] 6.0 docs need a general update

The docs point to the "legacy" way of doing things with the old Spa.Extensions library.

We revamped the way this works in 6.0 to leverage the same architecture followed by frontend frameworks to integrate with backends. The docs for angular and react should reflect that and cover the common use cases.

There is content here with details on how this works.

We should cover the following:

• How it works in development. (Frontend Dev Proxy proxies requests to ASP.NET Core)
• How it works in production. (Apps are just static files sitting in your wwwroot folder).

The important files on the ClientApp:

• proxy.conf.js (angular) to configure the webpack dev proxy (link to https://github.com/chimurai/http-proxy-middleware#options for all options)
• setupProxy.js (react) to configure the webpack dev proxy (same as angular).

Development modes:

• Launch frontend and backend simultaneously by launching the backend (as you are used to)
• Launch frontend and backend separately (by launching the frontend manually with npm run start) and avoid having to start the frontend server every time.

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Document Details

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• ID: 32a18270-91cd-2a02-2eb8-2c8b1ed03737
• Version Independent ID: c58d57e5-a0c2-201d-f6dd-076ad5971fcb
• Content: Use the Angular project template with ASP.NET Core
• Content Source: aspnetcore/client-side/spa/angular.md
• Product: aspnet-core
• Technology: aspnetcore-clientside
• GitHub Login: @SteveSandersonMS
• Microsoft Alias: stevesa