Rework URL schema

[baitando]

Currently there is a category (only it does exist right now) and the publishing date part of the URL. It would be better to remove both parts.

• The initial publish date is not beneficial, because most likely there will be no blog post with the same title resp. slug. Also a post could be updated. In case of an update, the URL with the initial publish date would be misleading.
• The category has no effect right now, because there is only one and there is no other category planned.
• The page should evolve, i.e. there could be some other parts beside the blog in the future. Therefore it could make sense to introduce a /blog path.
• Change URL of data privay policy to /data-privacy-policy.

It is important to take care of redirect rules. All previously existing URLs should permanently redirect to the new URL of a blog post.