Expand all leading tabs in markdown files

Given how problematic leading tabs in markdown files can be,
we have expanded all leading tabs in markdown files by:

```sh
find * -type f -name '*.md' -print |
  while read -r file; do
    rm -f "$file.tmp" ;
    expand < "$file" > "$file.tmp" ;
    mv -f -v "$file.tmp" "$file" ;
  done
```

Given the previous changes to `bin/pandoc-wrapper.sh` and other recent
commits and recent pull requests, the were were **NO** changes to
any HTML files.  This is a **good thing** as the expression goes.

No there are no leading tabs in any markdown files.
A check to catch any re-introduction of leading tabs
into any markdown file is on the way.

BTW: We do not have anything against leading tabs in files,
even though we have had to deal with their impact on forming
HTML from markdown files (as well as the calc source code).
It is just that this is one of those cases where tabs conflict
(sometimes caused by people wanting to change the classic
8-space tab model) with a desired outcome.

Perform `make www` to perform the above and test under macOS.

next/guidelines.md

@@ -421,7 +421,7 @@ Therefore do not assume the system has a
 [windows.h](https://en.wikipedia.org/wiki/Windows.h) include file:
 
 ``` <!---c-->
-    #include <windows.h>	/* we dislike this include */
+    #include <windows.h>        /* we dislike this include */
 ```
 
 Unless you are cramped for space, or unless you are entering the
@@ -467,9 +467,9 @@ Don't forget that the building of your program should be done
 
 ```
     prog: prog.c
-	#echo this next line requires data from standard input
-	cat > prog.c
-	${CC} prog.c -o prog
+        #echo this next line requires data from standard input
+        cat > prog.c
+        ${CC} prog.c -o prog
 ```
 
 However, you can do something cute such as making your program
@@ -479,8 +479,8 @@ For example, one could use the build instructions:
 
 ```
     prog: prog.c
-	${CC} prog.c -DNON_HUMAN_COMPILE -o prog
-	@echo "See remarks section about alternate ways to compile"
+        ${CC} prog.c -DNON_HUMAN_COMPILE -o prog
+        @echo "See remarks section about alternate ways to compile"
 ```
 
 and then include special notes in the program "remarks" for
@@ -642,9 +642,9 @@ Instead of unescaped octets, you should use \octal or \hex escapes:
 
 ``` <!---c-->
 
-	      /* 123456789 123456789 123456789 123456 */
+              /* 123456789 123456789 123456789 123456 */
     char *foo = "This string is 36 octets in length \263";
-	  /* This octet requires 4 octets of source ^^^^ */
+          /* This octet requires 4 octets of source ^^^^ */
     if (strlen(foo) == 36) printf("foo is 36 octets plus a final NUL\n");
     else printf("This code should not print this message\n");
 ```
@@ -707,7 +707,7 @@ to your "how to build" make compile line.  For example:
 
 ```
     prog: prog.c
-	${CC} prog.c -arch i386 -o prog
+        ${CC} prog.c -arch i386 -o prog
 ```
 
 Be even more creative!