Docs: Add headings so copy as markdown button shows up

[haleychaas]

Summary

We recently added the "Copy as Markdown" component to the docs. It is tied to the table of contents component, so in order to show up, all pages must have a table of contents. I've added headings to the pages that don't so that they do now, and therefore have the magical markdown button ✨ .

Preview here: https://docs-slack-d-branch-for-iox4s7.herokuapp.com/tools/bolt-js

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (b46454d) to head (e5a0500).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2796   +/-   ##
=======================================
  Coverage   93.45%   93.45%           
=======================================
  Files          37       37           
  Lines        7689     7689           
  Branches      669      669           
=======================================
  Hits         7186     7186           
  Misses        498      498           
  Partials        5        5           

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