refactor: rename projects to opensource-projects

Frees /products/ for the product page and makes the open-source archive
explicit, rather than leaving two sections that both look like "projects".

- Move the directory in both locales and update the 18 internal links
  pointing into it.
- Rework helix for the GitHub layout: server/ and web/ are directories in
  a single repository there, not the two separate repositories Gitea had.
  The "fork both repositories" step and the section wording follow.
- Correct helix's stated Java version to 21, which is what the build
  actually targets.

docs/en-gb/opensource-projects/onixbyte-toolbox/index.mdx

@@ -0,0 +1,54 @@
+---
+title: OnixByte Toolbox
+---
+
+import { Tabs, Tab } from "@rspress/core/theme"
+
+## Introduction
+
+OnixByte Toolbox is a Development Kit for Java applications that provides a set of convenient, reusable tools for writing code efficiently. It is designed and maintained by OnixByte as a monorepo containing multiple purpose-built libraries, each addressing a specific development need — from common utilities and cryptography to mathematical computation and identity generation.
+
+The toolkit is available on Maven Central under the `com.onixbyte` group ID. Each module is independently versioned and can be included in any Java 17+ project.
+
+## Modules
+
+| Module             | Artifact ID          | Description                                                               |
+|--------------------|----------------------|---------------------------------------------------------------------------|
+| Common Toolbox     | `common-toolbox`     | Everyday utilities — AES, hashing, Base64, collections, branching, ranges |
+| Crypto Toolbox     | `crypto-toolbox`     | Public/private key loading for RSA and ECDSA algorithms                   |
+| Math Toolbox       | `math-toolbox`       | Statistical calculation, percentile analysis, quartile computation        |
+| Tuple              | `tuple`              | Generic 2-tuple and 3-tuple with immutable variants                       |
+| Identity Generator | `identity-generator` | Snowflake-style and sequential UUID identity generation                   |
+| Version Catalogue  | `version-catalogue`  | Centralised Gradle version catalogue for OnixByte projects                |
+
+## Installation
+
+Add the desired module as a dependency in your build config:
+
+<Tabs>
+  <Tab label="Gradle with Kotlin DSL">
+    ```kotlin title="build.gradle.kts"
+    dependencies {
+      implementation("com.onixbyte:$artefactId:$version")
+    }
+    ```
+  </Tab>
+
+  <Tab label="Maven">
+    ```xml title="pom.xml"
+    <dependency>
+      <groupId>com.onixbyte</groupId>
+      <artifactId>${artefactId}</artifactId>
+      <version>${version}</version>
+    </dependency>
+    ```
+  </Tab>
+</Tabs>
+
+## Requirements
+
+- Java 17 or later
+
+## License
+
+OnixByte Toolbox is open-source software released under the MIT Licence.