[Docs] Getting started and Use Cases

[marklysze]

Why are these changes needed?

Getting Started / Quickstart and Use Cases documentation.

Notes:

• Placeholders for code using code files TBD (currently using --SNIPPET: conversableagentchat.py)
• Folder structure for code files TBD (currently within the same folder)
• Using index.md, TBD whether this should be .mdx or other
• Depends on single-agent chat functionality (Support for a single agent run #469, Add global initiate chat and make it easy to use single agent #538)
• Needs to be updated in line with top-level initiate chat functionality ([Feature Request]: Separate Initiate Chat and Single Agent Chat #493, Add global initiate chat and make it easy to use single agent #538)

Outlines:

Related issue number

Closes #466
Closes #635

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.