Is this page a policy or guideline, or the source for all policies and guidelines?
It is none of those. It is a non-binding description of some of the fundamental principles, begun by User:Neutrality in 2005 as a simple introduction for new users. For comparison, WP:NPOV, WP:NOT and WP:IAR were first written down on Wikipedia in 2001, and WP:NOR and WP:V were written in 2003.
Even though it has nothing to do with the Five Pillars of Islam, won't Muslim people be offended anyway?
Muslim editors commenting here have confirmed that there is nothing offensive in this. The Arabic Wikipedia uses exactly the same words to title their version of this essay. The words "five pillars" are not inherently sacred; the same words might be used by Muslims in everyday speech, such as to describe architectural elements in a building.
Does this page list every single important principle?
No. It does not discuss the importance of using common sense, not charging money to readers, cooperating with Wikipedias in other languages, the desirability of making pages accessible to people with disabilities or limited internet access, or any number of other principles that the community has identified as important over the years.
