Fix #11353: Add v2 unsupported note

Fix issue #11353 by adding a note to the v2.3.2 docs saying that we no
longer support this version of Bootstrap. Provides a link to the latest
docs, i.e. our homepage.
Merge pull request #11404 from twbs/add-warning-to-old-docs

Fix #11353: Add v2 unsupported note