Unify tags and categories

[lukaszstolarczuk]

Blog posts, announcements, videos (and other sections with tags and categories) should unify naming and grouping.

Right now (especially blog posts) are "randomly" added to categories. I believe this should be updated and unified, e.g., for easier usage like on /pmemstream: https://pmem.io/pmemstream#blog-entries

We may easily point to all posts/announcements/videos/... on selected microsties.

When the unification is done, we should also update the archetypes with proposed categories... or maybe we should list somewhere what's proposed (e.g. in README or in CONTRIBUTING files).