[editorial] Move `supplementary-guidelines` to be under `docs`

[chalin]

PR #1234 got me thinking:

• Why not put supplementary-guidelines under docs?

If the content is non-normative, then the README of that section (and the README of docs) can just state that. By putting the section under docs, it conveniently gets published on the OTel website.

References:

• https://github.com/open-telemetry/semantic-conventions/tree/main/docs
• https://github.com/open-telemetry/semantic-conventions/tree/main/supplementary-guidelines
• Move http migration guide to supplementary-guidelines directory #1234

Another thought: this move might be an opportunity to rename the section directory to non-normative-guidelines or just non-normative -- although you might also feel that the current name is fine.

[joaopgrassi]

Why not put supplementary-guidelines under docs

Sounds like a good idea to me!

[chalin]

Why not put supplementary-guidelines under docs

Btw, this was actually one of the "other to-dos" from:

• Adjust to move Semantic Conventions out of OTel spec opentelemetry.io#2721

Sounds like a good idea to me!

Done, PTAL at:

• [editorial] Move supplementary-guidelines into docs #1284

[chalin]

/cc @jsuereth