Fixed some accessibility, and moved homepage things around.

[MaanavD]

Addresses #21638

Changed install to genai, moved genai section up, accessibility changes in docs/install.

Still todo (another PR, soon to come):

• Change theme for accessibility
• Change videos linked on homepage
• Update homepage content to more closely reflect current ORT status.

Preview at : https://maanavd.github.io/onnxruntime/