Input fields should not have pre-entered values, instead they should display a placeholder

On several pages, the search field is filled with text instead of a placeholder. This is not a problem, but it results in an additional unindent action to clear the input field before use.

• https://www.xwiki.org/xwiki/bin/view/documentation/
• https://extensions.xwiki.org/xwiki/bin/view/Extension/ (same with "Contribute Extension" field the login user will have pre-typed input)