[Docs] Database preparation: Clarify why database client is installed on Gitea server...

[Docs] Database preparation: Clarify why database client is installed on Gitea server (#12833)

* Database preparation: Clarify why database client is installed on Gitea server

The database client program (`mysql` or `psql`) is used to test database
connection, but Gitea use Go's database driver to do the same.

Also, rename "server part" and "client part" to "server application" and
"client program" to disambiguate.

* Update docs/content/doc/installation/database-preparation.en-us.md

Co-authored-by: 6543 <6543@obermui.de>