Generalize disabled nav links CSS rules

[julien-deramond]

Description

Most of the time, our .nav-links are really links and don't need :disabled rules. However, we provide JavaScript examples (e.g. with .nav-tabs) where we use <button>s that can need :disabled rules.

This PR suggest a kind of defensive approach where the disabled state is defined globally for all .nav-links using .disabled and :disabled selectors.

It allows us to remove the other rules defining the colors.

⚠️ Rules defining transparent "color" to background and border were also removed because I couldn't notice use cases where it was actually changing anything. I haven't found anything in the history, but it can be put back if you are aware about any use cases where it's mandatory.

Please double-check different complex use cases to avoid any regressions since the documentation does not reflect all possibilities.

Type of changes

• Enhancement (non-breaking change which adds functionality)

Checklist

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

Live previews

• https://deploy-preview-38774--twbs-bootstrap.netlify.app/docs/5.3/components/navs-tabs/

Related issues

Linked to #38765 but doesn't take into account #38765 (comment)

[patrickhlauke]

I looked at this purely for the #38765 scenario, and this fix here works great. not had a chance to go through all possible complex variations