Add Table of Contents

[tigrannajaryan]

• Add Makefile and markdown-toc to generate Table of Content.
• Replace <h*> tag by hashtags in markdown so that markdown-toc tool can pick them up.
• Used make markdown-toc target to generate it for this change.

Resolves https://github.com/open-telemetry/opamp-spec/issues/72

[tigrannajaryan]

The new github action passed: https://github.com/tigrannajaryan/opamp-spec/runs/5872599273?check_suite_focus=true