Update existing GCP icons using updated official GCP icons:
see https://cloud.google.com/icons. This includes changes to GCP nodes
and examples shown on the website.

Note, generic icons are updated with the corresponding new generic icon,
except "ai-platform-data-labeling-service.png" since it now has its
own unique icon.

Closes #554.

refactor

* Add EMQX as a On-Prem Queue entry

* Add larger EMQX icon

* Basic support for C4 model primitives.

* Use the "rect" shape for nodes

With the record shape we used before, graphviz would trip over
edges that set constraint=False.

* Adopt C4 terminology: Rename Dependency -> Relationship

* Adopt C4 terminology: Rename type -> technology

* Extract a shared C4Node

This makes the code more DRY, but also allows to add company-
specific extensions more easily. One need we have is to slightly
adapt the terminology. At Spotify, we happen to call `Container`
a `Component` for example. This is now easier to implement on top
of the shared `C4Node`.

* Add "C4" shield to the README

* Document how to produce a C4 diagram

* Try to make the custom doc page (#284)

* [Fix] link syntax

* [Fix] Generate the doc for custom outside the provider loop + Remove custom from providers list.

* [Fix] Add custom menu in side bar

* docs(Custom): Add more examples (#284)

* Edges support

* Documentation and additional style attribute

* Better example and review changes

* Fix xlabel

* Fix docs

* Add Edge default properties

* Fix edge docs

Co-authored-by: Andrew Selivanov <a_selivanov@wargaming.net>

* feat: add custom node class

* docs: add example about custom node

* docs: improve example about custom node usage

* docs: fix variable name