Add tip to installation instructions regarding disabling Cardinal

This is also reflected in the HPC instructions. Finally, improve spacing for warning message when Cardinal is disabled.

config/check_deps.mk

@@ -11,7 +11,7 @@ ENABLE_CARDINAL      := yes
 CARDINAL_CONTENT     := $(shell ls $(CARDINAL_DIR) 2> /dev/null)
 
 ifeq ($(CARDINAL_CONTENT),)
-  $(warning $n"Cardinal does not seem to be available. If usage of Cardinal is desired within FENIX, make sure that either the submodule is checked out$nor that CARDINAL_DIR points to a location with the Cardinal source. $n In the meantime, FENIX will be built without Cardinal.")
+  $(warning $n"Cardinal does not seem to be available. If usage of Cardinal is desired within FENIX, make sure that either the submodule is checked out$nor that CARDINAL_DIR points to a location with the Cardinal source. $n$nIn the meantime, FENIX will be built without Cardinal.")
   ENABLE_CARDINAL    := no
 else
   $(info FENIX is using Cardinal from     $(CARDINAL_DIR))

doc/content/getting_started/installation.md

@@ -92,15 +92,22 @@ export OPENMC_CROSS_SECTIONS=~/projects/FENIX/cross_sections/cross_sections.xml
 
 !alert! warning
 This variable +must+ be set in your environment anytime you wish to run FENIX input files that
-utilize the OpenMC functionality! This can be done either using `export` on the command line,
-or placing this command within a shell config file (`.bashrc`, `.zshrc`, etc.).
+utilize the OpenMC functionality within Cardinal! This can be done either using `export` on the
+command line, or placing this command within a shell config file (`.bashrc`, `.zshrc`, etc.).
 !alert-end!
 
 !alert! note
 The copies of MOOSE, TMAP8, and Cardinal provided with FENIX have been fully tested against the current
 FENIX version, and is guaranteed to work with all current FENIX tests.
 !alert-end!
 
+!alert! tip title=Disabling Cardinal
+Cardinal is an optional dependency of FENIX and can be disabled to speed up build times, if Cardinal
+capabilities are not needed. This can be done by simply not downloading the Cardinal submodule
+(i.e., not performing the `git submodule update --init cardinal` command as well as the Cardinal
+dependency download commands above.)
+!alert-end!
+
 Once all dependencies have been downloaded, FENIX can be compiled and tested:
 
 ```bash