Convert top-bar table to bulleted list in docs/01-understanding-github-web-structure.md

[taylorarndt]

Convert the table under 'The Global Navigation Bar (always present)' to a bulleted list to improve scanability and accessibility. Suggested format in the issue thread. File: docs/01-understanding-github-web-structure.md

[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.