[Docs] Docs updates (#14824)
### Description
Staged: https://faxu.github.io/onnxruntime/docs/
Changes:
- ~~New dedicated section on sidebar for
[Mobile](https://faxu.github.io/onnxruntime/docs/mobile/) and
[Web](https://faxu.github.io/onnxruntime/docs/web/)~~ New dedicated
pages for
[Mobile](https://faxu.github.io/onnxruntime/docs/get-started/with-mobile.html)
and
[Web](https://faxu.github.io/onnxruntime/docs/get-started/with-web.html)
under Getting Started section
- [Transformers
optimizer](https://faxu.github.io/onnxruntime/docs/performance/transformers-optimization.html)
documentation
- Move contrib op section from "Custom Operators" page to ["Contrib
operators"
page](https://faxu.github.io/onnxruntime/docs/reference/operators/ContribOperators.html)
- Update [releases
page](https://faxu.github.io/onnxruntime/docs/reference/releases-servicing.html)
