add doc for v5.1: Deploy -> Operate TiKV

[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 Deploy -> Operate TiKV

Any related PRs or issues?

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

Which version does your change affect?

v5.1

[marsishandsome]

@andylokandy @zz-jason @dcalvin @iosmanthus PTAL

[marsishandsome]

@andylokandy PTAL

[marsishandsome]

@zz-jason PTAL