Removed relative addresses in announcements (#4267)

This will be used later for the joincpi.org website to directly pull announcements from this repo

Author: Vinceyou1

announcements/2021 Feb.mdx

@@ -7,4 +7,4 @@ order: 4
 
 All problems from the
 [USACO 2021 February Contest](http://www.usaco.org/index.php?page=feb21results)
-have been added to the USACO problem table [here](/general/usaco-monthlies).
+have been added to the USACO problem table [here](https://www.usaco.guide/general/usaco-monthlies).

announcements/Groups Open To Public.mdx

@@ -9,4 +9,4 @@ We're opening access to [USACO Guide Groups](/groups) to everyone! Groups is a
 learning management system that allows assigning problems, posting
 announcements, and tracking progress with a leaderboard. Now, you're able to
 create your own group and invite your friends -- go to
-[the groups page](/groups) to get started.
+[the groups page](https://www.usaco.guide/groups) to get started.

announcements/Open Source.mdx

@@ -8,5 +8,5 @@ order: 3
 If this project has helped you, consider
 [giving us a star on Github](https://github.com/cpinitiative/usaco-guide)! As a
 fully open source project, we depend on community members like you to improve
-our product. Check out the [contributing](/general/contributing) module to learn
+our product. Check out the [contributing](https://usaco.guide/general/contributing) module to learn
 how you can help make the USACO Guide better.