DOC-12540 avx2 requirement vector search

[ggray-cb]

This PR documents that when running Vector Search in 7.7 and later on an x86 platform, the CPU must have the AVX2 instructions.

This PR updates two pages:

• What's New has an entry covering the use of AVX2, and mentions the AVX2 requirement.
• System Resource Requirements adds the AVX2 requirement to the processor requirements table, plus a subheading that explains the requirement in more detail.