List of tags as taxonomy pages

[sn00py1310]

As an expansion to #24 it would be great if the tags had taxonomy pages.
Including:

• an overview over all tags
• on each tags term site an overview of all documents with a specific tag

If the tags should be under the docs directory, you would need to add something like that to your hugo.toml

# See https://github.com/gohugoio/hugo/issues/11846
# See https://discourse.gohugo.io/t/re-new-permalinks-config-in-0-115-x-permalinks-term/45218

[taxonomies]
  docs-tags = "docs-tags"

[permalinks.taxonomy]
   docs-tags = "docs/tags/"
[permalinks.term]
   docs-tags = "docs/tags/:slug"