Specification: left-nav contains subsection entries

[chalin]

Visit https://opentelemetry.io/docs/reference/specification/ on desktop and you'll see the following: the left-nav (red box in screenshot below) shows flattened collection of pages from arbitrary sections, as you can see from the screenshot below.

What we need to do to fix this is to ensure that every section has an index (README) page.

Tasks:

• [editorial] Ensure common section has a README opentelemetry-specification#2479

Related:

• Spec: promote "Attribute Limits" up one heading level #1279
• Contributes to Integrate the OTel specification into the website #906

[chalin]

This has mostly be addressed, I'm going to close this in favor of tracking the remaining fixes via:

• [editorial] Ensure every section has a README opentelemetry-specification#2558