[Feedback]: tidying up the kerberos introduction

### Page URL

https://documentation.ubuntu.com/server/explanation/intro-to/kerberos/#introduction-to-kerberos

### Select an option

- [ ] I found what I was looking for
- [ ] I couldn't find what I was looking for
- [x] I found the information, but it was incorrect
- [x] I found the information, but it was incomplete
- [ ] I found the information, but it was confusing
- [ ] I encountered a technical issue (e.g., broken link, image not loading)

### Issue Description

a) This paragraph at the top should be removed, because the actual installation and configuration is done elsewhere:
> This section covers installation and configuration of a Kerberos server, and some example client configurations.

b) The first sentence reads in an odd way, and is incorrect in one aspect (it's not a system, but a protocol):
> Kerberos is a network authentication system based on the principal of a trusted third party.

I suggest:
"Kerberos is a network authentication protocol based on shared secrets that provides strong authentication for client/server applications and users."

c) The resources section should drop the reference to Matrix or IRC, that is something we have for all pages in the footer.

Open the Kerberos introduction at the page URL and review the introduction and resources sections. Remove the installation/configuration paragraph, replace the inaccurate opening sentence with the suggested wording, and remove the Matrix or IRC reference; done means all three requested documentation changes are reflected on the page.

Written by the indexing model from the issue text.