docs: Document versions in README.md (#130)

* docs: Inform on versions available

* docs: Update front-most gap-versions

* docs: Fix bullet punctuation

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* docs: Update README.md

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: limakzi

README.md

@@ -6,10 +6,10 @@
 
 ```mermaid
     graph RL
-        V1[gap-docker:4.13.1-bare]      --> T
-        V2[gap-docker:4.13.1-full]      --> T
-        V3[gap-docker:4.13.1-slim]      --> T
-        V4[gap-docker:4.13.1-buildfull] --> T
+        V1[gap-docker:4.15.1-bare]      --> T
+        V2[gap-docker:4.15.1-full]      --> T
+        V3[gap-docker:4.15.1-slim]      --> T
+        V4[gap-docker:4.15.1-buildfull] --> T
         T --> V
         V --> G
         
@@ -18,18 +18,36 @@
         T@{ shape: processes, label: "bare/slim/buildfull/full"}
 ```
 
+We have the following variants[^1]:
+
+* **`bare`** - Core compiled from source with only the `PackageManager` package installed. Required packages are loaded via `InstallRequiredPackages()` — no optional packages are compiled. This is the _smallest image_, suitable for minimal or custom setups where you want full control over which packages to add.
+
+* **`slim`** - Includes additional runtime libraries needed by optional packages. Suitable for users who want to _selectively compile_ only the packages they need. All deposited `GAP` packages (i.e. those distributed in the release archives) are included, but those requiring compilation _have not been_ compiled. Some system packages required to compile GAP packages are also installed in the container.
+
+* **`full`** - Contains _all packages compiled_ via `BuildPackages.sh --parallel`. It is the largest image and the most comprehensive variant. Recommended for most users.
+
+* **`jupyter-gap`** - A separate image based on the `full` variant. Adds `JupyterLab` and the `gap-kernel`. Exposes port `8888` and launches notebook as the default command.
+
+Besides that we have:
+
+* **`buildfull`** - An _internal build stage_ used to compile all packages for the `full` image. Referenced in the build pipeline but not intended for direct use.
+
+---
+
+## Usage
+
 * Run a container:
 
 ```
-docker run -it ghcr.io/gap-system/gap:4.13.1-full
+docker run -it ghcr.io/gap-system/gap:4.15.1-full
 ```
 
 ```
-docker run -it ghcr.io/gap-system/gap:4.14.0-full
+docker run -it ghcr.io/gap-system/gap:4.15.0-full
 ```
 
 ```
-docker run -it ghcr.io/gap-system/gap:4.13.1-bare
+docker run -it ghcr.io/gap-system/gap:4.14.0-bare
 ```
 
 ```
@@ -45,3 +63,5 @@ Although it was completely rewritten, this repository consists of various ideas
 
 [1]: https://github.com/james-d-mitchell/gap-docker-minimal
 [2]: https://github.com/stertooy/gda-image
+
+[^1]: All variants are based on `Ubuntu 22.04` LTS.