Update syntax.md with recent feature additions, document old features

[Oberon00]

This fixes a few issues with syntax.md:

• There was no update to syntax.md for
  • Support span/resource/metric types #14 which added the "type" field
  • Update semconv tool with stability fields and MD render. #43 which added the "stability" field.
  • This PR does NOT document the units type introduced in Add unit semantic convention generator #21 (because I'm not familiar with it, maybe @justinfoote can create a follow-up).
• The tag, deprecated and sampling_relevant fields that the tool supported from the beginning were never documented.
• The description of extends was lacking.

The above was originally written by @thisthat, thank you!

• The relative link to the spec's attributes description broke after moving the tool here.
• Also, some minor formatting improvements & subsection restructuring.