Add docs for version_compile_* variables (#6649)
Add docs for version_compile_* variables (#6649)

* Add docs for version_compile_* variables

Related PR: https://github.com/pingcap/tidb/pull/28958

```
 MySQL  127.0.0.1:4000 ssl  test  SQL > SHOW VARIABLES LIKE 'version\_compile\_%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| version_compile_machine | amd64 |
| version_compile_os      | linux |
+-------------------------+-------+
2 rows in set, 1 warning (0.0028 sec)
```

* Update based on review
pingcap/docs,pingcap/docs,pingcap/docs