A review comment in JDK-8351881 suggested adding a new factory method HtmlTree.IMG to avoid running into future bugs where the "alt" attribute is missing for images.

A certain pattern emerges when looking at uses of HtmlTag.IMG, we will clearly benefit from having a new static helper method.

They all look very similar to the following:
```
HtmlTree.of(HtmlTag.IMG)
                                .put(HtmlAttr.SRC, pathToRoot.resolve(DocPaths.RESOURCE_FILES)
                                        .resolve(DocPaths.CLIPBOARD_SVG).getPath())
                                .put(HtmlAttr.ALT, copyUrlText)
```

No tests are required as part of this change, it has no impact on the JDK doc build at present.