Add images to the webapp pages.

docs/assets/webapp/changelog.png

docs/assets/webapp/contact.png

docs/assets/webapp/credits.png

docs/assets/webapp/data.png

docs/assets/webapp/download-resources.png

docs/assets/webapp/feedback.png

docs/assets/webapp/filters.png

docs/assets/webapp/footer.png

docs/assets/webapp/geo-resources.png

docs/assets/webapp/header.png

docs/assets/webapp/homepage.png

docs/assets/webapp/other-resources.png

docs/assets/webapp/partners.png

docs/assets/webapp/reuses.png

docs/assets/webapp/search.png

docs/assets/webapp/sidemenu.png

docs/assets/webapp/suggestion.png

docs/assets/webapp/web-app-project-structure.png

docs/components/custom-apps/web-app/changelog.md

docs/components/custom-apps/web-app/contact.md