Use 'Understanding' consistently across TiDB & TiKV Development Guide

[sunxiaoguang]

Looks like TiDB Dev Guide is using 'Understand TiDB' however TiKV Dev Guide uses 'Understanding TiKV' instead.

Looked into Rust book, and it uses 'Understanding XXX' consistently. Therefore I suggest we use 'understanding' as well.

[sunxiaoguang]

We can rename SUMMARY.md first and rename directory after finished initial version of all the chapters to avoid conflicting people's in progress work.

[tisonkun]

Make senses. I'll mark it as help wanted for anyone does the actual work. We're not in a hurry for this.