Add past projects to the build

[zanemit]

Relates to pull request #269

Add a Past projects section under Projects. The project_materials.yml file was updated to reflect the new desired page organisation.

- file: projects/docs/past_projects_overview.md
  title: Past projects
  section:
  - file: projects/past-projects/sea_level_rise.ipynb
    title: Sea Level Rise
  - file: projects/past-projects/ocean_acidification.ipynb
    title: Ocean Acidification
  - file: projects/past-projects/ENSO.ipynb
    title: The Impact of ENSO on Precipitation and Temperature
  - file: projects/past-projects/precipitation.ipynb
    title: Regional Precipitation Variability and Extreme events
  - file: projects/past-projects/heatwaves.ipynb
    title: "Heatwaves: Assessing the Dynamic Interactions of the Atmosphere and Land"
  - file: projects/past-projects/wildfires.ipynb
    title: Monitoring and Mapping Wildfires Using Satellite Data
  - file: projects/past-projects/albedo.ipynb
    title: "Changes in Land Cover: Albedo and Carbon Sequestration"