document S3 cache more fully; fixes #12289
docs: use relative URLs instead of site.baseurl
Corrected tag for Alert JS animation example from <a> to <button>

Also removed href attribute and added type="button"
Merge pull request #12307 from twbs/bye-site-baseurl

docs: use relative URLs instead of site.baseurl
Merge pull request #12303 from twbs/doc-s3

document S3 cache more fully
fix #12314
fix #12311
fix markup typo, duh
upgrade #callout-navbar-overflow from info to warning
nitpick phrasing