Overhaul README.md (#56)

* Use GitHub Flavored Markdown for admonitions

* Add syntax highlighting to code blocks

* Minor raw formatting changes

* Add 'Metric' header to PageSpeed Insights table

* Fix typo

* Minor wording changes

* Add link to launchd page on Wikipedia

* Overhaul README.md

* Add table of contents

* Reorganise About section

* Edit PageSpeed table

* Add instructions for streaming container logs

* Rework Instances section

* Add note about Compose plugin installation

* Add link to instance monitoring

* Move info on architecture specific images into an admonition

* Change NGINX reverse proxy admonition to important

* Change Redlib external links admonition to a note

* Edit section headings

* Fix anchor link under Deployment section

* Add comparison with Libreddit

* Update ToC