Install a github action to run Lighthouse on PR

## I want to do this:

Install a github action to run Lighthouse on PR
These are the coursework repos: https://github.com/CodeYourFuture/Table-of-Contents
These are the template repos: https://github.com/CodeYourFuture/CYF-Coursework-Template

_Given a coursework repo_
When a trainee opens a pull request
And the coursework contains or produces complete HTML pages
Then Lighthouse is run
And the score is reported on the pull request

When the score is less than 100 on Accessibility
Then the PR is set to draft

## Here’s why I want to do it:

We have found that Lighthouse is really helpful for helping trainees evaluate their own code
Setting a lighthouse report on PR is useful for trainees and volunteers to check basic front end quality

## Here’s how it serves our goals:
- automation frees up volunteers for meaningful technical mentorship
- we want trainees to write good code and this will help them and help them see how to fix things
- Also listed on Syllabus https://github.com/CodeYourFuture/syllabus/issues/529

## This is how much time I can put towards this change:
- I can spend 3 hours over 4 weeks testing this for a devops person and merging it in to relevant repos, but I cannot teach someone to do this and then test it for them -- I need devops people to take this forward as I don't have capacity

## This is the help I need from others to get this done (if any):

- a devops grad or volunteer to put this in place
- @Dedekind561 to keep an eye on how it will work with his github actions

Commencez par examiner les repositories de coursework Table-of-Contents et CYF-Coursework-Template, ainsi que leurs workflows GitHub Actions existants. Déterminez comment Lighthouse peut s’exécuter sur les pull requests et en rendre le score, puis vérifiez que le seuil d’accessibilité entraîne la conversion de la PR en brouillon. Le travail est considéré comme terminé lorsque le workflow fonctionne pour les repositories de coursework et est compatible avec la configuration GitHub Actions existante.

Rédigé par le modèle d'indexation à partir du texte de l'issue.