Add HTML validator pipeline to landing page pipeline

[coffeemakingtoaster]

Problem

The main branch and therefore all PRs to the main branch should validate the html to prevent the main branch from being in a broken stage. The pipeline should be executed on push and pull request to main branch.

Possible Solution

Create workflow using https://github.com/marketplace/actions/html5-validator

[LarsFlieger]

This is a good idea! However we should ask @Zoe-Bot which tool she wants to use to build the landing page. Maybe she is picking some frameworks as well. (Static site or react)

[coffeemakingtoaster]

@Zoe-Bot set this to active and update the project board based on the framework you plan on using (if any)