doc: css: Add style for C code, files and man pages

Use monospace and disable italic for various C code, files and man
pages. For is used as the default for sphinx
(sphinx/themes/basic/static/basic.css.jinja).

Examples of code where it is used:

    <a class="reference internal"
    href="api_c_tests.html#struct-tst-test"><span class="std std-ref">struct
    tst_test</span></a>p

    <a class="extlink-master reference external"
    href="https://github.com/linux-test-project/ltp/blob/master/include/tst_test.h">include/tst_test.h</a>

    <a class="extlink-kernel_tree reference external"
    href="https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/stat.h">include/uapi/linux/stat.h</a>

    <a class="manpage reference external"
    href="https://man7.org/linux/man-pages/man2/setrlimit.2.html">setrlimit(2)</a>

Link: https://lore.kernel.org/ltp/20260114122611.372930-1-pvorel@suse.cz/
Reviewed-by: Andrea Cervesato <andrea.cervesato@suse.com>
Signed-off-by: Petr Vorel <pvorel@suse.cz>

Author: pevik

doc/_static/custom.css

@@ -8,3 +8,9 @@
 .rst-content table td ul {
     margin-bottom: 0;
 }
+
+/* monospace and disable italic for C code, files and man pages */
+a.extlink-master, a.extlink-kernel_tree, a.extlink-kselftest, a.extlink-shell_lib, a.manpage, .std.std-ref {
+    font-style: normal;
+    font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", Courier, monospace;
+}