doc/README.md - add "tip" alert styling

Add "tip" alert styling (what in Docbook XML is called "an admonition")
to information about Ninja in an ordered list (which is what markdown
has here instead of procedures).

Signed-off-by: Zac Dover <[email protected]>

README.md

@@ -92,19 +92,20 @@ To build Ceph, follow this procedure:
 
     ``ninja``
 
-   [Ninja](https://ninja-build.org/) is the build system used by the Ceph
-   project to build test builds.  The number of jobs used by `ninja` is 
-   derived from the number of CPU cores of the building host if unspecified. 
-   Use the `-j` option to limit the job number if build jobs are running 
-   out of memory. If you attempt to run `ninja` and receive a message that 
-   reads `g++: fatal error: Killed signal terminated program cc1plus`, then 
-   you have run out of memory.
-
-   Using the `-j` option with an argument appropriate to the hardware on which 
-   the `ninja` command is run is expected to result in a successful build. For
-   example, to limit the job number to 3, run the command `ninja -j 3`. On
-   average, each `ninja` job run in parallel needs approximately 2.5 GiB 
-   of RAM.
+   > [!TIP]
+   > [Ninja](https://ninja-build.org/) is the build system used by the Ceph
+   > project to build test builds.  The number of jobs used by `ninja` is 
+   > derived from the number of CPU cores of the building host if unspecified. 
+   > Use the `-j` option to limit the job number if build jobs are running 
+   > out of memory. If you attempt to run `ninja` and receive a message that 
+   > reads `g++: fatal error: Killed signal terminated program cc1plus`, then 
+   > you have run out of memory.
+   >
+   > Using the `-j` option with an argument appropriate to the hardware on
+   > which the `ninja` command is run is expected to result in a successful
+   > build. For example, to limit the job number to 3, run the command `ninja
+   > -j 3`. On average, each `ninja` job run in parallel needs approximately
+   > 2.5 GiB of RAM.
 
 This documentation assumes that your build directory is a subdirectory of the
 `ceph.git` checkout. If the build directory is located elsewhere, point