Migration guide: .panel-title's closest replacement is .card-header

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Use variables for .progress base colors
/fyi @mdo

Refs #18535
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Refs #18535

Address IE10 in browser compatibility docs

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Add RubyGems section to "Download" docs

Closes #17291 by finishing the last remaining task.
Also reorders the sections in the "Download" docs.

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Introduce $nav-item-margin variable
/fyi @mdo

Introduce $card-deck-margin variable
/fyi @mdo; happy to tweak the var names if desired

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I don't wanna get diabetes. 
Obey The Mickens!: https://vimeo.com/111122950#t=14m9s

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remove generated content value for progress

How did the linters miss this?

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Fix anchor link

Remove extraneous character

Docs: Replace `data-src` attribute with `src` in highlighted code

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Docs Responsive breakpoints from em to px

Using px rather than em values

Should close https://github.com/twbs/bootstrap/issues/18556

Rename 2 navbar variables to comply with updated naming convention

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as it's currently not displayed, and (in my opinion) would look fugly if
it did appear...

progress bar for IE10+/Edge (border radius, use ::-ms-fill, variant mixins, striped)

clarify lack of support for striped and animated striped bars in
IE10+/Edge

Remove fully supported features from IE9 limitations docs

If these are supported, why bother mentioning them in a section that's about communicating what's not supported?
These rows made sense when the section also covered IE8 (http://getbootstrap.com/getting-started/#support-ie8-ie9 ),
which doesn't support these features, but we no longer care about IE8 in v4.

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Docs: Include major version in `npm install bootstrap` commands
Future-proofing for when Bootstrap v5 eventually exists.
Refs #18520

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Add Wall of Browser Bugs entry for #18504

https://connect.microsoft.com/IE/feedback/details/2110930/edge-ie11-gradient-background-image-on-tr-is-only-applied-to-first-td

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