[docs] Update rrdiagram.jar download instructions (yugabyte#2156)

* Update JAR file download instructions

The link for `rrdiagram.jar` will soon be outdated.  Change the download
instructions to suggest getting the latest file through two methods:
manually going to the releases page or running a shell command that
dynamically figures out the latest release.

* Update gitignore rrdiagram pattern

The pattern for ignoring rrdiagram jar files does not capture certain
cases such as `rrdiagram-0.9.4-yb-1.jar`.  Update the pattern with one
more wildcard character to handle this case.

* Fix markdown errors in docs README

The docs README has issues with word wrapping code blocks and
incorrectly linking sections in different repositories.  Fix these
issues.

* Reword note for alternatively building jar

As @stevebang commented, reword the note about alternatively building
the `rrdiagram.jar` file.

.gitignore

@@ -89,4 +89,4 @@ test_failures.json
 .metadata/
 
 # Diagram generator for docs
-rrdiagram.jar*
+rrdiagram*.jar*

docs/README.md

@@ -137,12 +137,18 @@ _Note: Modifications will typically be added to the `ysql` or `ycql` subdirector
     ./content/latest/api/ycql/syntax_resources/commands/copy_from,copy_to,copy_option.diagram.md
     ```
 
-4. Download the RRDiagram JAR file (`rrdiagram.jar`) using the following command.
-
-    ```bash
-     wget https://github.com/YugaByte/RRDiagram/releases/download/0.9.4/rrdiagram.jar
-    ```
-    _Note: Alternatively build manually as described in the [Build](#build) section below (and move/rename the resulting jar from the target folder)._
+4. Download the latest RRDiagram JAR file (`rrdiagram.jar`).  You can find it on the [release
+   page](https://github.com/YugaByte/RRDiagram/releases/), or you can try running the following
+   command.
+
+   ```bash
+   wget $(curl -s https://api.github.com/repos/YugaByte/RRDiagram/releases/latest \
+          | grep browser_download_url | cut -d \" -f 4)
+   ```
+
+   _Note: Alternatively, you can manually build the jar file as described in the [build
+   section](https://github.com/YugaByte/RRDiagram/README.md#build) of the `RRDiagram` repo (and
+   move/rename the resulting jar from the target folder)._
 
 5. Run the diagram generator using the following command: