docs(connectors): add docs for secure TLS connexion

[mathias-vandaele]

Description

closes camunda/connectors#2806

For self-managed connectors, we did not provide an easy way of setting up a custom certificate for zeebe client, this PR fixes this.

When deploying using those env variables, zeebe will now use the given certificate to set up a secure connection to the zeebe client

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

[github-actions]

👋 🤖 🤔 Hello! Did you make your changes in all the right places?

These files were changed only in docs/. You might want to duplicate these changes in versioned_docs/version-8.5/.

• docs/self-managed/connectors-deployment/connectors-configuration.md

You may have done this intentionally, but we wanted to point it out in case you didn't. You can read more about the versioning within our docs in our documentation guidelines.

[ev-codes]

@mathias-vandaele thanks, looks good!

I went ahead and capitalized terms that needed it (e.g., Zeebe). In public-facing docs, it's best to follow convention for easier reading 😄

[akeller]

Hey there! Please be sure to fill out the PR description and add labels. This helps the DevEx team, specifically the technical writers, know how and when to review and release the docs. Also, be sure to link back to Product Hub epics or team board issues so we have more context about this change, as we often try to do some validation (is this change properly backported, are there other areas that need to be changed as well, etc.).