Various editorial fixes on term definitions and references

[tidoust]

A few updates that were initially meant to fix broken fragments reported by the Link Checker to be able to publish an updated Working Draft. This degenerated a bit in various improvements to internal references to better take advantage of ReSpec. Changes:

• Added links to GitHub repository and issue tracker near the top of the spec
• Adjusted terminology section to reference terms actually used by the spec and to use ReSpec new data-cite mechanism: https://github.com/w3c/respec/wiki/data--cite
• Turned all references to IDL terms into links to the definition of the term
• Used <var> throughout procedures where referring to variables
• Dropped explicit IDs from the spec, typically those that ReSpec handles automatically
• Similarly, simplified references to internal definition to just make use of ReSpec with a simple <a> instead of hardcoding the fragment anchor
• Fixed broken fragment references