Add Swarm section to the Docker Provider Documentation

[dduportal]

What does this PR do?

This PR aims at adding a Swarm section in the Docker provider documentation,
introducing the following changes:

• [Docker provider documentation] Elaborate that Swarm Mode is supported + elaborate and moving upfront the "Routing Configuration" for Docker standalone
• [Docker provider documentation] Add a section "Routing Configuration with Labels" for Docker standalone to elaborate the concept (as the quickstart is not really explicit about the "label" magic), and mention other tools that could be used if Docker is used in background
• [Docker provider documentation] Add a section "Port Detection" for Docker standalone to explain the port detection mechanism
• [Docker provider documentation] Add a section "Docker API Access" for Docker standalone with the content around security moved here (and adapted to recent publications as it was for Traefik v1)
• [Docker provider documentation] Add a full section "Docker Swarm Mode", mirroring the 3 sub sections added to standalone (labels, port and api access) to elaborate on the swarm mode specifics.
• [Docker provider & Docker routing documentations] Minor adaptations to adapt to the new sections introduction (moving configuration examples, adding internal links, etc.)

Motivation

• We want Docker Swarm users to be able to learn how to use Traefik in a single location.
• We want Docker users to fully understand how Traefik interacts with Docker and/or Swarm.
• We want to leverage the amount of community issues on these topics by improving the documentation based on the raising voices from community of users.
• Follows up Dashboard example with swarm #5795
• Closes Docker swarm configuration ignored with "port is missing" #5732
• Related to https://community.containo.us/t/dashboard-gives-me-an-error-bad-gateway/1883/2 and https://community.containo.us/tags/docker-swarm

More

• [ ] Added/updated tests
• Added/updated documentation

Additional Notes

[dtomcej]

Looking good, just a few comments!

[dtomcej]

LGTM

[mpl]

just a quick pass about english/typoes. I haven't given any thought about the document's structure and contents.

[ldez]

LGTM

[jbdoumenjou]

LGTM

requested by mpl