Add Tenzir v6 migration guide

[mavam]

🔍 Problem

• Customers need a clear migration path before Tenzir v6 makes the new engine the default.
• The temporary compatibility controls and pipeline changes were not documented in one high-visibility guide.
• Existing docs still contained copyable examples and references for the removed generic from and to operators.

🛠️ Solution

• Add a Tenzir v6 migration guide with migration modes, compatibility controls, a checklist, v6 execution patterns, migration reference tables, and before/after examples.
• Add the guide to Guides → Get Started and cross-link it from the affected operator references.
• Clarify subpipelines as a first-class v6 pattern in the programs reference.
• Replace legacy from/to examples across docs with named source and sink operators, and remove the obsolete from/to reference pages/cards.

💬 Review

• This PR is stacked on topic/new-executor, because the migration guide links to and builds on the new executor/operator documentation there.
• Please focus on the migration reference for completeness and on the exact wording around neo: false, // neo, and //neo:false.
• Please also review the chosen replacements for the removed generic from and to examples, especially AMQP, SQS, TCP, and UDP examples.

_{✅ Closes TNZ-523}

[github-actions]

📦 Preview  ·  🟠 Still propagating

_{The deploy completed, but the public URL has not served build 64d414b yet. Check again in a minute.}

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: a0c8eba6f6

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: f29ba118ae

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 126f755e27

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".