[BFCL Blog] BFCL V3 wording refine (#681)

Changes: 
- Be clear that BFCL V3 consist of existing datasets from BFCL V1 and
V2. BFCL V3 introduces a new category, i.e. multi-turn, multi-step into
the mix.
- Route users to earlier blog posts if they are not familiar with the
concept of function calling, and evaluations in single-turn single-step
scenarios. These knowledge are foundational and prerequisite for BFCL V3
blog.