[doc] Update "Get Started" page for ORT web (#19568)
### Description
This PR re-arranges documentation of ORT web.
Current doc: https://onnxruntime.ai/docs/
Change preview: https://fs-eire.github.io/onnxruntime/docs/
- split `Doc -> Get Started -> JavaScript` from single page into
multiple pages, to make it easier to split from web, node and
react-native
- remove `Doc -> Get Started -> with Web`. The page was a few links and
now they are moved to other places
- add a few contents into `Doc -> Get Started -> JavaScript -> Web`. Now
this is the main page for ORT web doc.
- rename `Tutorials -> deploy on Web` to `Tutorials -> Web`. "deploy" is
confusing as it's not the accurate term to describe the content.
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### Discussions:
TBD
=================================================================
### ORT web documentation work item list:
- [ ] Update `Get Started` page for ORT web at onnxruntime.ai **( this
PR )**
- to make it easier for users to navigate
- [ ] Update `Tutorials` page for ORT web at onnxruntime.ai
- The purpose of this page is unclear. It contains descriptive
information and step-by-step instructions on an E2E examples. It can be
a good Blog, but not good for documentation, which usually has a
well-structured content.
- The [index
page](https://fs-eire.github.io/onnxruntime/docs/tutorials/web/) has
duplicated contents with its child page ["Build a web application with
ONNX
Runtime"](https://fs-eire.github.io/onnxruntime/docs/tutorials/web/build-web-app.html)
- Some contents are out-of-dated.
- [ ] Update [`API Usage -
SessionOptions`](https://github.com/microsoft/onnxruntime-inference-examples/tree/main/js/api-usage_session-options)
on onnxruntime-inference-examples repo.
- Add explaination and examples if necessary for all new WebGPU session
options.
- [ ] Update [`API usage - ort.env
flags`](https://github.com/microsoft/onnxruntime-inference-examples/blob/main/js/api-usage_ort-env-flags)
on onnxruntime-inference-examples repo.
- Add all newly introduced flags.
