Re-levelization: move non-docs content, rename content→docs

For better URLs, the content folder has been renamed 'docs' and all
other files have been moved up a level. Also, non-docs images have been
moved to the static folder at the top level where they belong.

Many relative paths had to be fixed to make this work.

static/img/logos/github.svg

@@ -1,26 +1,4 @@
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