Fix `sitemap: false` instruction not working

[dmitrio95]

Some pages in this theme use the sitemap: false instruction to prevent their inclusion to the sitemap.xml file. Some examples of such pages:

• 404 error page
• Redirection example

However this instruction doesn't seem to work. The current sitemap.xml of this theme's website contains the links to 404.html, redirect-page, search, despite all of them have the sitemap: false instruction. Apparently the only currently working way to exclude the page from the sitemap is setting the sitemap.exclude variable, like it is done here.

This pull requests corrects the behavior of the sitemap: false instruction thus allowing excluding a page from a sitemap both via the sitemap and sitemap.exclude variables.