Fix doc links (#884)

[dtchepak]

• move /docs to /help
• remove dates from doc names
• move docs to sub-directories and rename to index.md so we get nice urls (e.g. help/example rather than help/example.html.
• use wget --spider to detect and fix broken links.
• fix search rendering. Note: dark mode does not play nicely with the GCSE input box1
• rearrange home page to put install links at the top