[editorial] Ensure every section has a README

[chalin]

Originally posted by @tigrannajaryan in #2479 (comment)

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).

• [editorial] Ensure common section has a README #2479
• [editorial] Add missing READMEs to section #2559
• [editorial] Ensure logs section has a README #2625
• [editorial] Ensure metrics subsections have a README #2629
• specification/schemas - [editorial] Ensure schemas section has a README, and other changes #3046
• [editorial] Ensure trace subsections have a README #3553
• [editorial] Add missing README.md to logs exporters folder #3936
  specification/logs/sdk_exporters - will leave as is for now. For details, see comment further below.

Useful command:

$ find specification -type d '!' -exec test -e "{}/README.md" ';' -print | grep -v /img

[chalin]

Only one spec folder is without a README:

$ find specification -type d '!' -exec test -e "{}/README.md" ';' -print | grep -v /img
specification/logs/sdk_exporters
$ ls specification/logs/sdk_exporters
stdout.md

But given that it contains only one exporter, I think that it's ok for now to display the Stdout exporter under the Logs parent section.

So I'm going to mark this as done, finally! ✨

[chalin]

Changed my mind about the logs exporters folder, in particular because it'll be a quick workaround for the following Hugo 0.123.8 issue: Subpage missing from section's page list when using 0.123.8. Reopening.