feat: show categories on posts (#973)

## Summary

* Add a `categories.html` partial that renders linked Hugo `categories`
terms with the same styling pattern used for tags.
* Include categories before tags on single pages.

## Motivation

Posts already show tags, but Hugo also supports `categories` as a
taxonomy. This resolves the requested single-page category display from
issue #620.

Closes #620

## Documentation

Documentation PR: gohugo-ananke/documentation#18

## Verification

* Compared `issues/620` against `development`; the branch is ahead by
only the intended two files.
* Confirmed `layouts/_partials/tags.html` already renders terms with
`.GetTerms "tags"`, and the new categories partial mirrors that approach
with `.GetTerms "categories"`.
* Could not run a local Hugo build because the execution container
cannot resolve `github.com` for repository cloning or dependency
installation.