Notify the docker image and the installation information in the back…

[pierre-yves-monnet]

…end-diagram-converter README.

Camunda Community Hub Pull Request Template

Description

Notify the docker image in the documentation for the backend converter

Additional context

It was not very clear that a docker image existed. In the backend documentation, the only mention is "how to build the application", so to install it, the first step seems to be to build it first.
But a docker image exists, ready to use, and it is better to start to mention it, before the build.

Testing your changes

Only documentation

Types of changes

• Bug fix (non-breaking change which fixes an existing open issue)
• New feature (non-breaking change which adds functionality to an extension)
• Breaking change (fix or feature that would cause existing functionality of an extension to change)
• [X ] Documentation update (changes made to an existing piece of documentation)

Checklist:

• My code adheres to the syntax used by this extension.
• My pull request requires a change to the documentation.
• [X ] I have updated the documentation accordingly.
• [X ] I have read the Camunda Community Hub documentation.
• [X ] I have read the Pull Request Process documentation.
• I have added or suggested tests to cover my changes suggested in this pull request.
• All new and existing CI/CD tests passed.
• I will /assign myself this issue, and add the relevant [issue labels] to it if they are not automatically generated by Probot.
• I will tag @camunda-community-hub/devrel in a new comment on this issue if 30 days have passed since my pull request was opened and I have not received a response from the extension's maintainer.

[github-actions]

Test Results

  26 files  +  26    26 suites  +26   2m 16s ⏱️ + 2m 16s
214 tests +214  214 ✔️ +214  0 💤 ±0  0 ❌ ±0 
332 runs  +332  332 ✔️ +332  0 💤 ±0  0 ❌ ±0 

Results for commit 052dc99. ± Comparison against base commit 6e67daa.

♻️ This comment has been updated with latest results.