Add documentation for credits, drafts and feeback features in the web app section

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

+# Credits
+
+This page lists the open source tools used by data.grandlyon.com.
+
+The list is fetched from the dedicated [credits](../../services/credits.md) service.
+
+For each credit the page displays the logo, the name and a short description. Each card (credit) is clickable and will open a new tab at the address of the website of the credit.

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

+# Drafts
+
+This page is dedicated to the users belonging to the `ghost-editors` group.
+
+The purpose of the page is to list non published articles. Each article is presented with its image cover, title and the last modification date and are clickable. Clicking on an article redirects to the article's page and so we are able to preview it as it will be displayed once published. Thereby we can anticipate any display bug before the publication of the article.

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

+# Feedback
+
+The goal of this feature is to provide users an easy way to give their opinion to the development team but also to submit any suggestions of improvment, problems or bugs they might have encountered.
+
+The feedback form is composed of a textarea for the feedback, a 'rating' field where the value can be `1 star`, `2 stars`, `3 stars` or `No opinion` and an optional email field if the user wants a reply from the team. If an email is provided the user has to accept the general term of use and the process of the information in order to sumbit its feedback.