[doc] add doc of `show backend config`

[DarvenDuan]

Versions

PR https://github.com/apache/doris/pull/36525 has implemented showing backend config by ShowStmt, add some docs.

• [x] dev
• [x] 3.0
• [ ] 2.1
• [ ] 2.0

Languages

• [x] Chinese
• [x] English