update for ORT Web build guide (#12247)
* update for ORT Web guide
I reordered a few things and added some more context.
I think we can also add more information on converting ONNX to ORT format fairly easily. maybe in the doc update next iteration :)
* Doc update - Following the github discussion
* Update web.md
Updated the doc - fixed the title hierarchy
Added a small comment regarding the final build products.
* Update web.md
Updated the titles "Build ONNX Runtime for web assembly" --> "Build ONNX Runtime web assembly artifacts"
Updated the "build NPM package" doc section - clarified the different sections of the build process
* Update web.md
Fixed a broken link
* Update web.md
Adding a section to clarify there is one last step for finalizing the build process
