related-projects: rename section

Libraries for server use -> Client libraries, Server utilities
related-projects: fix wrong section id

introduction -> related_projects
related-projects: use "Groonga" notation
related-projects: remove needless parentheses

It is identified in the section name
related-projects: fix the number of categories
Merge pull request #12 from naoa/fix-related-projects

Fix related projects

Patch by Naoya Murakami. Thanks!!!