Add more links from quick start pages to API docs

[pchalasani]

In the quick-start docs and other places, wherever we refer to a Langroid class, variable or method, we should make it a hyperlink to the code document-page (and section) where it is defined, e.g.

[`DocChatAgent`][langroid.agent.special.doc_chat_agent.DocChatAgent]

in
https://github.com/langroid/langroid/blob/main/docs/quick-start/chat-agent-docs.md#answer-questions-from-a-set-of-urls

[Mohannadcse]

for a variable [use_functions_api][langroid.agent.chat_agent.ChatAgentConfig]??

[pchalasani]

We can start with linking classes and method names, then do more granular.