Convert 'Secondary navigation' paragraph to bulleted list in docs/01-understanding-github-web-structure.md

[taylorarndt]

I feel the "Secondary navigation (repository pages only)" paragraph is hard to scan quickly. Please consider converting that short paragraph and the inline tab list into a simple bulleted list for better readability and screen-reader scanning.

Suggested replacement (example) in docs/01-understanding-github-web-structure.md:

Secondary navigation (repository pages only)

When you are inside a repository, a second navigation bar appears below the global bar. This contains the repository's tabs:

• Code
• Issues
• Pull requests
• Actions
• Projects
• Wiki
• Security
• Insights
• Settings

Why:

• Short bulleted lists are faster to scan and nicer for keyboard/voice navigation than an inline code row.
• Improves clarity for screen reader users when skimming the page structure.

@accesswatch @maintainers: Thoughts? If we agree, please implement the list replacement and open a PR.

File: docs/01-understanding-github-web-structure.md
Section: "Secondary navigation (repository pages only)"

[taylorarndt]

Closing — what I thought were tables were actually ASCII art diagrams. These have been addressed in PR #168 which converts them to SVG images with prose descriptions below each one.