Update landing page

[razvand]

Prerequisite Checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Updated relevant documentation (if needed).

Description of changes

Move landing page in chapters/ directory to make it easy to reference chapters. Improve the contents to reference chapters and present contribution guides.

[teodutu]

It only hit me after merging: you need to change the landing page path in config.yaml: https://github.com/open-education-hub/oer-template/blob/main/config.yaml#L33

[razvand]

It only hit me after merging: you need to change the landing page path in config.yaml: https://github.com/open-education-hub/oer-template/blob/main/config.yaml#L33

I will do a force push.