Automate lists-of-conventions in README files or remove them

[lmolkova]

We have several files listing available conventions that are usually out-of-date:

• https://github.com/open-telemetry/semantic-conventions/blob/main/docs/README.md
• https://github.com/open-telemetry/semantic-conventions/blob/main/docs/general/metrics.md
• https://github.com/open-telemetry/semantic-conventions/blob/main/docs/messaging/README.md
• https://github.com/open-telemetry/semantic-conventions/blob/main/docs/database/README.md
• ...

We can try autogenerating such sections or avoid listing them at all.