index: expand content to explain what makes pmOS unique

reopens !310 (closed)

The current front page is sparse and the What others say section is more of an info dump than a section helping people better understand what postmarketOS is, does, and what makes it unique—even though they are great quotes for that purpose.

By rearranging the quotes, adding guiding headings, and adding surrounding paragraphs, I try to better explain this, and tell the story of pmOS.

This also embeds the mission statement into the flow.

Closes: #178

As already mentioned in #178, the upstream projects visual is a mere brain dump of a placeholder. The idea is more or less to have a visualisation of how well pmOS is connected and to whom.

The general structure, I think, works quite well, but not happy with every paragraph, either—especially the first paragraph, that mostly is the way it is due to the constraint that I wanted the UI list to still be at the top.

The structure I meant to achieve:

The first section is supposed to be more about what pmOS achieves, what you can expect from it, and why it's created. It should also cater a bit to users, without creating a “service provider” expectation—you should feel the community-driven and continual evolution aspect of postmarketOS—, nor a iOS/Android replacement expectation. And still, I try to avoid negative statements like “did not yet achieve Android-level support” in this section. I would also like to avoid open questions like the segue “Why” to the mission statement, so a better idea is welcome.

The second section should drive home on the community aspect, the collaboration inside the project, and outside it, underline the importance of giving back to upstream, and emphasise the social aspect of community-driven projects.

The last part is a call to action. Maybe, there is for more of them in between.