Various updates to eco-kde-org website
Changes include:
- Replaced "BE4FOSS" with "Sustainable Goal" in top panel as well as at the top of the page
- Added "Say hello to KDE Eco" video
- Removed old link to forum, updated it with link to Eco teams at invent.kde.org, added Gitlab icon
- Added description of Sustainable Software Goal
- Removed information about BE4FOSS funding given that the project ended 31 March 2023
- Shortened text related to BE4FOSS, updated the text to indicate that the project has ended