Merge branch '160-add-mattermost-to-list-of-custom-apps' into 'main'

Resolve "Add Mattermost to list of custom apps"

Closes #160

See merge request !138

frontend/public/custom/assets/mattermost.svg

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frontend/public/custom/markdown/mattermost.md

+---
+title: 'Mattermost'
+tileExcerpt: 'Team collaboration and communication app'
+---
+
+## Introduction
+
+> Work together effectively with real-time communication, file and code snippet sharing, in-line code syntax highlighting, and workflow automation purpose-built for technical teams. From the [official website](https://mattermost.com/)