[SEO] use site title as default, not subtitle

[m1rm]

First: I started using this theme and it is very nice!

I then noticed, that the home page does not have the meta description defined. Upon investigating the code, I found:

https://github.com/CaiJimmy/hugo-theme-stack/blob/master/layouts/partials/data/description.html

which uses the subtilte as default for the meta description.

I think, in consequence, "hugo managed sites" do not have it set.

Other themes use the title as description if the description attribute is not set in the front matter. I hereby suggest to do the same.

If we like the idea, I am happy to create a PR.

[CaiJimmy]

Sorry, could you please clarify what "Hugo managed sites" are? I don't understand why they can't have a subtitle field.