Installation guide as main page for new users rather than download page.

OK, after trying to transform in several ways, I came up with a minimal change that should help to work from.

This MR makes a link for new users to the installation guide directly from the FAQ rather than going to the download page.

We do not not have control on what to download for most of users. We cannot provide hash, we do not even know for sure what version will be provided on distributions. At the end we release source, and distributions will have their own release schedule (even if we do it ourselves, it will not be following release schedule).

Also installation of dependencies when installing from source will change depending on the distribution installed on. Not based on the version of BuildStream.

So users should follow recommendations from the installation guide rather being told to install specific versions.

Users that need to access to the download page are package maintainers a power-users. There are still links from the FAQ. But it is made clear that this is for people who know what they want.

From the download page, there is also a link to the installation guide. Because even power users might need guidance for installation. However in order to avoid repeating installation instructions, which are very long, we keep everything on the installation guide.

On the installation guide, which quite big, the table of content is rather discouraging. So I have written an introduction that explains what sections are important depending on the case.

I have put it in "WIP", not because I intend to modify things in the MR, but because I expect discussion.