Update Linux installation instructions

[RogueScholar]

The instructions for installation on Linux systems employed command syntax that is now deprecated and often failed to provide natural contextual links to external sources of information that cover more complex tasks in greater detail. This PR corrects those issues, along with the following:

• Add/update parameters to HTML iframe tag to better address
  • Safely embedding external content (allow, referrerpolicy, sandbox)
  • Web accessibility "a11y" best practices (aria-label, name, role, title)
  • Graceful handling of network constraints (loading)
  • Conversion of old HTML4 rubrics to modern CSS3 declarations (allowfullscreen -> allow="fullscreen;", frameborder="0" -> style="border-collapse: collapse; border-style: hidden;")
• Convert links to Markdown "reference style" for deduplication and easier updating of targets

[hizzlekizzle]

This looks great, thanks! I've been meaning to update the linux instructions for a while, but the accessibility and webdev stuff was not on my radar at all, so even better :)