docs: update document string and site (#1211)

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

README.md

@@ -158,6 +158,7 @@ You can always cleanup test database by removing `/tmp/greptimedb`.
 - GreptimeDB [User Guide](https://docs.greptime.com/user-guide/concepts.html)
 - GreptimeDB [Developer
   Guide](https://docs.greptime.com/developer-guide/overview.html)
+- GreptimeDB [internal code document](greptimedb.rs)
 
 ### Dashboard
 - [The dashboard UI for GreptimeDB](https://github.com/GreptimeTeam/dashboard)

src/cmd/src/bin/greptime.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#![doc = include_str!("../../../../README.md")]
+
 use std::fmt;
 
 use clap::Parser;