feat(docs): restructure documentation with new framework and templates

- Archive old docs to docs-old/ for reference
- Create new top-down documentation structure:
  * Platform Overview: purpose, audience, product structure
  * Components: individual platform components (Forgejo, Kubernetes, Backstage)
  * Getting Started: onboarding and quick start guides
  * Operations: deployment, monitoring, troubleshooting
  * Governance: ADRs, project history, compliance
- Add DOCUMENTATION-GUIDE.md with writing guidelines and templates
- Add component template (TEMPLATE.md) for consistent documentation
- Simplify root README and move technical docs to doc/ folder
- Update test configuration:
  * Exclude legacy content from markdown linting
  * Relax HTML validation for theme-generated content
  * Skip link checking for legacy content in test:links
  * Keep 'task test' clean for technical writers (100% pass)
  * Add 'task test:full' with comprehensive link checking
- Update home page with corrected markdown syntax
- Fix internal links in archived content

BREAKING CHANGE: Documentation structure changed from flat to hierarchical top-down approach

content/en/docs-old/decisions/_adr-template.md

@@ -0,0 +1,67 @@
+<!-- we need to disable MD025, because we use the different heading "ADR Template" in the homepage (see above) than it is foreseen in the template -->
+<!-- markdownlint-disable-next-line MD025 -->
+# {short title, representative of solved problem and found solution}
+
+## Context and Problem Statement
+
+{Describe the context and problem statement, e.g., in free form using two to three sentences or in the form of an illustrative story. You may want to articulate the problem in form of a question and add links to collaboration boards or issue management systems.}
+
+<!-- This is an optional element. Feel free to remove. -->
+## Decision Drivers
+
+* {decision driver 1, e.g., a force, facing concern, …}
+* {decision driver 2, e.g., a force, facing concern, …}
+* … <!-- numbers of drivers can vary -->
+
+## Considered Options
+
+* {title of option 1}
+* {title of option 2}
+* {title of option 3}
+* … <!-- numbers of options can vary -->
+
+## Decision Outcome
+
+Chosen option: "{title of option 1}", because {justification. e.g., only option, which meets k.o. criterion decision driver | which resolves force {force} | … | comes out best (see below)}.
+
+<!-- This is an optional element. Feel free to remove. -->
+### Consequences
+
+* Good, because {positive consequence, e.g., improvement of one or more desired qualities, …}
+* Bad, because {negative consequence, e.g., compromising one or more desired qualities, …}
+* … <!-- numbers of consequences can vary -->
+
+<!-- This is an optional element. Feel free to remove. -->
+### Confirmation
+
+{Describe how the implementation of/compliance with the ADR can/will be confirmed. Are the design that was decided for and its implementation in line with the decision made? E.g., a design/code review or a test with a library such as ArchUnit can help validate this. Not that although we classify this element as optional, it is included in many ADRs.}
+
+<!-- This is an optional element. Feel free to remove. -->
+## Pros and Cons of the Options
+
+### {title of option 1}
+
+<!-- This is an optional element. Feel free to remove. -->
+{example | description | pointer to more information | …}
+
+* Good, because {argument a}
+* Good, because {argument b}
+<!-- use "neutral" if the given argument weights neither for good nor bad -->
+* Neutral, because {argument c}
+* Bad, because {argument d}
+* … <!-- numbers of pros and cons can vary -->
+
+### {title of other option}
+
+{example | description | pointer to more information | …}
+
+* Good, because {argument a}
+* Good, because {argument b}
+* Neutral, because {argument c}
+* Bad, because {argument d}
+* …
+
+<!-- This is an optional element. Feel free to remove. -->
+## More Information
+
+{You might want to provide additional evidence/confidence for the decision outcome here and/or document the team agreement on the decision and/or define when/how this decision the decision should be realized and if/when it should be re-visited. Links to other decisions and resources might appear here as well.}