Improving the YARP documentation organization

As part of the migration to ASP.NET docs, it has been a forcing function to make me look at the docs. The following are some organization changes to improve the docs based on the new TOC. This asks to split up some of the topics under Configuration as they included both config & extensibility content - splitting those docs into 2 will make them shorter and more relevant based on how the customer is using YARP.

- [ ] /core/fundamentals/servers/yarp/transforms - split this doc at the `Extensibility` heading, and put the second half as `Transforms` under the extensibility node
- [ ] core/fundamentals/servers/yarp/load-balancing - split this doc at the `Extensibility` heading, and put the second half as `Load Balancing` under the extensibility node
- [ ] core/fundamentals/servers/yarp/dests-health-checks - split out the 2 Extensibility sections into one doc under the extensibility node in the TOC.

Another suggestion may to further split up the /core/fundamentals/servers/yarp/transforms doc into 3:
- Transforms overview
- Request Transforms
- Response Transforms and Response Trailers
And make it another layer in the TOC under `Proxy Features` as its quite a mammoth doc, and the split may make it easier from a TOC perspective to understand what you can do with transforms.

### Page URL

https://learn.microsoft.com/en-us/aspnet/core/fundamentals/servers/yarp/getting-started?view=aspnetcore-9.0

### Content source URL

https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/fundamentals/servers/yarp/getting-started.md

### Document ID

75f2623e-0c7d-3392-9b37-7a2d86163591

### Article author

@samsp-msft

### Metadata

* ID: 75f2623e-0c7d-3392-9b37-7a2d86163591
* Service: **aspnet-core**
* Sub-service: **fundamentals**