Deprecate amplify,backbone,bootstrap,d3

packages/deprecated/README:

```
These packages are no longer actively maintained by MDG.  Seek
community alternatives instead.

Note that these packages still exist in atmosphere, and you can still
build a package that depends on one of them by specifying an explicit
package version.
```

Nothing technically prevents us from publishing further versions of these packages (like the ones in packages/non-core).
Rename “showdown” to “markdown”

If we want to change markdown implementations in the future, we’ll bump the major version of the package.
Rename “showdown” to “markdown”

If we want to change markdown implementations in the future, we’ll bump the major version of the package.
Merge branch 'deprecate-packages' into devel