refactor: CHARTS-6395 Restructure examples folder

[khanguslee]

• Nested all existing examples into an examples/charts directory
• Converted appropriate links to use github tree instead of blob as we weren't directly linking to a file
• Update all links to github to use the nested examples/charts directory
• Update all links to codesandbox to use the nested examples/charts directory
• Renamed all readme.md files to README.md for consistency
• Updated gif in main README.md file to show new MongoDB branding 😄

Preview of gif:

Note:

All links will not work on this PR (for now) as the links are updated for master branch.