Docs: replace twbs/bootstrap-npm-starter by twbs/examples/tree/main/icons-font

[julien-deramond]

Bootstrap 4 is EOL in few weeks.
It means that https://github.com/twbs/bootstrap-npm-starter shouldn't be referenced anymore in our latest documentation. And that it could probably by archived.

This PR suggested that all references to https://github.com/twbs/bootstrap-npm-starter are changed to use the closest already existing example we have... and it seems to be https://github.com/twbs/examples/tree/main/icons-font.

This PR also suggest to update the Starter template example page to add references to our other examples.

This version tries not to modify too much wording and references by keeping the notion of "npm started" even if our example is rather named "Bootstrap Icons", but we can go further.
/cc @mdo

Type of changes

• Refactoring (non-breaking change)

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

Live previews

• https://deploy-preview-37671--twbs-bootstrap.netlify.app/
  • Link in the footer
  • Link in the homepage
• Starter template example
• Modified callout used in:
  • Customize > Sass > Variable defaults
  • Getting started > Contribute > Using npm scripts
  • Getting started > Download > Package managers

[mdo]

I think https://github.com/twbs/examples/tree/main/sass-js or https://github.com/twbs/examples/tree/main/sass-js-esm is a better fit here instead of the Icons example which doesn't compile Bootstrap's source Sass code at all. Thoughts?

[julien-deramond]

I can replace in this PR https://github.com/twbs/examples/tree/main/icons-font with https://github.com/twbs/examples/tree/main/sass-js as suggested and in a second time recreate the starter template in https://github.com/twbs/examples which is mayble more complete?

[mdo]

Updated it to replace the links as mentioned. Also updated the starter template's CSS, so this is now dependent on the icon link addition, and fixes the .text-dark instance. Once that merges, this can merge.