Replace headers in test .md files with bold text

[yash-chowdhary]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [x] Other, please explain: Modification of markdown files of test sites

Fixes #740

What is the rationale for this request?
This PR replaces headers with bold text in an attempt to reduce noise (HTML content) generated.

What changes did you make? (Give an overview)
Replaced headers with **bold** text across various .md files in the test/ directory.

Provide some example code that this change will affect:
Screenshot of the affected site shown below:

Is there anything you'd like reviewers to focus on?
On running npm run updatetest, there were several (6-7) images that were flagged as "modified", but the image content itself remained unchanged. Should these "modified" images also be included in the PR?

Proposed commit message: (wrap lines at 72 characters)
Replace headers in test .md files with bold text

[yamgent]

Thanks for cleaning this up.

I think these files can also be cleaned up:

• test/functional/test_site/testAntiFOUCStyles.md
• test/functional/test_site/testPlantUML.md
• test/functional/test_site/testThumbnails.md
• test/functional/test_site_algolia_plugin/index.md

[yamgent]

Thanks, it is generally much cleaner now.

For the record, normally we would ask you to clean up the commits by squashing them into reasonable chunks (for this PR, a reasonable organization would just be a single commit "Replace headers in test .md files with bold text"). In interest of time I will just squash it for you now, but do take note of this in future PRs.