Update redirects in shortcut dirs

Description

This PR suggests to change some HTML redirections in gh-pages branch in order to avoid having redirections to v3 or v4. Maybe it'll help with SEO as well.

⚠️ I've only updated what already existed. IMO we should either have all shortcuts for each sections in the docs or nothing at all (and so remove /examples, javascript, etc. in this branch at the root level). Thoughts?

Deleted redirections because doesn't exist anymore:

• https://getbootstrap.com/browser-bugs/ (redirected to v4.5)
• https://getbootstrap.com/css/ (redirected to 3.4)
• https://getbootstrap.com/examples/jumbotron-narrow/ (redirected to 3.4)
• https://getbootstrap.com/examples/justified-nav/ (redirected to 3.4)
• https://getbootstrap.com/examples/navbar-fixed-top (redirected to 3.4)
• https://getbootstrap.com/examples/static-top (redirected to 3.4)
• https://getbootstrap.com/examples/navbar (redirected to 3.4)
• https://getbootstrap.com/examples/non-responsive (redirected to 3.4)
• https://getbootstrap.com/examples/offcanvas (redirected to 3.4)
• https://getbootstrap.com/examples/signin (redirected to 3.4)
• https://getbootstrap.com/examples/theme (redirected to 3.4)
• https://getbootstrap.com/examples/tooltip-viewport (redirected to 3.4)
• https://getbootstrap.com/javascript (redirected to 3.4)

Modified redirections to v5.3:

• https://getbootstrap.com/customize/ (redirected to 3.4)
• https://getbootstrap.com/examples/blog (redirected to 3.4)
• https://getbootstrap.com/examples/carousel (redirected to 3.4)
• https://getbootstrap.com/examples/cover (redirected to 3.4)
• https://getbootstrap.com/examples/dashboard (redirected to 3.4)
• https://getbootstrap.com/examples/grid (redirected to 3.4)
• https://getbootstrap.com/examples/jumbotron (redirected to 3.4)
• https://getbootstrap.com/examples/starter-template (redirected to 3.4)
• https://getbootstrap.com/examples/sticky-footer-navbar (redirected to 3.4)
• https://getbootstrap.com/examples/sticky-footer (redirected to 3.4)

Type of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• All new and existing tests passed