Document asdf, .tool-versions purpose, usage etc

We use asdf's .tool-versions files in a number of places (GDK, gitlab, shell, pages etc) yet we don't document anywhere:

• What asdf is
• What asdf's advantages, disadvantages/shortcomings etc are
• What the file does exactly
• How are the files managed
• How the GDK's .tool-versions file is different to the other (spoiler, it's generated dynamically from support/asdf-combine)