add docs for v5.1: reference -> Query Layers + TiUP + FAQs

[marsishandsome]

What is changed?

TiKV's official website is planned to be refactored as discussed in https://github.com/tikv/website/issues/227.

This PR covers Reference -> Query Layers + TiUP + FAQs

This PR references to

• https://tikv.org/docs/4.0/reference/query-layers/
• https://tikv.org/docs/4.0/reference/faq/

Any related PRs or issues?

https://github.com/tikv/website/issues/227.

Which version does your change affect?

v5.1

[marsishandsome]

@andylokandy @zz-jason @iosmanthus PTAL