[editorial] Ensure common section has a README

[chalin]

• Contributes to fixing Specification: left-nav contains subsection entries opentelemetry.io#1280 by doing the following:
  • Promotes common/common.md to be the section's index file (common/README.md)
• Renames #attributes anchors to their canonical name #attribute (singular), under the Common page

/cc @austinlparker @tigrannajaryan @carlosalberto

[tigrannajaryan]

The common.md content is probably not a great fit for a README. I think we can allow this PR since it will result in a better than currently situation on the website, but we probably will need to re-work the README after that to make the content more suitable as an introduction to what a "common" section in the specification supposedly should be.

Question: do we need a README file in every sub-directory under the "specification" directory? Most sub-directories currently don't have one.

[chalin]

The common.md content is probably not a great fit for a README. I think we can allow this PR since it will result in a better than currently situation on the website

Great!

Question: do we need a README file in every sub-directory under the "specification" directory? Most sub-directories currently don't have one.

Yes, in order to fix

• Specification: left-nav contains subsection entries opentelemetry.io#1280

every section will need a README. I'm available to help here: e.g., I submit a first PR that adds a README (with only a title) to each section that doesn't have such a file. That'll act as a placeholder that can be built upon via subsequent PRs. WDYT?

[tigrannajaryan]

every section will need a README. I'm available to help here: e.g., I submit a first PR that adds a README (with only a title) to each section that doesn't have such a file. That'll act as a placeholder that can be built upon via subsequent PRs. WDYT?

Yes, sounds good. Some sub-directories have good candidates for README (e.g. most that have an "overview.md" should work as README).