Add monitoring and alerting for about.gitlab.com

Problem to solve

Currently (as far as I am aware) there is no automatic monitoring for the about.gitlab.com domain to make sure it is available.

Recently we had an outage of the handbook, gitlab-com/www-gitlab-com#7897 (closed), where it took 46min for someone to pick up that all the handbook pages were missing and report it.

I am wondering if someone can help me understand what options are available for putting monitoring in place for key pages on about.gitlab.com to check that they are giving a 2xx response.

Requirements

1. Monitor key URLs on the about.gitlab.com domain (see below)
2. Send an alert message to the #handbook-escalation Slack channel when a failure is detected.

Key pages

The following are examples of URLs we'd want to monitor (based on high traffic and build process differences)

1. https://about.gitlab.com/
2. https://about.gitlab.com/blog/
3. https://about.gitlab.com/blog/2020/05/20/creating-a-dark-ui-for-gitlabs-web-ide/
4. https://about.gitlab.com/releases/
5. https://about.gitlab.com/releases/categories/releases/
6. https://about.gitlab.com/releases/2020/05/22/gitlab-13-0-released/
7. https://about.gitlab.com/direction/
8. https://about.gitlab.com/company/team/
9. https://about.gitlab.com/handbook/
10. https://about.gitlab.com/handbook/values/
11. https://about.gitlab.com/handbook/total-rewards/compensation/compensation-calculator/calculator/
12. https://about.gitlab.com/handbook/engineering/projects/