Add simple usage instructions where appropriate

[bernhardreiter]

As https://oasis-open.github.io/csaf-documentation/tools.html directly links to https://github.com/csaf-poc/csaf_distribution/blob/main/docs/csaf_downloader.md it makes sense to add two things to the page:

a) a hint of simple usage example, aka "just use it with your domain". Rationale: the full technical documentation does not need to be read to use the downloader, it is more for reference and special cases.
b) a recommendation to use the prebuild binaries. Rationale: The home page with the instructions cannot be directly reached from the page and it documents too many variants.

For me this is part of maintenance for #550.

And we should check if the situation for a) and b) is similar to other tools from our set.