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.

markdown.md

@@ -119,7 +119,7 @@ Please do **NOT** start code blocks at the left-hand edge.
 
 ````
 ```
-This code block is NOT indented                   		<=== no thank you
+This code block is NOT indented                                 <=== no thank you
 ```
 ````
 
@@ -161,7 +161,7 @@ For example:
 
 ````
 ```
-        Please do **NOT** indent with ASCII tabs.		<=== no thank you
+        Please do **NOT** indent with ASCII tabs.               <=== no thank you
 
     The top level starts with a 4 ASCII space indent.
 
@@ -180,7 +180,7 @@ We request that (fenced) markdown code blocks **NOT** specify a language.
 For example:
 
 ````
-```c								<=== no thank you
+```c                                                            <=== no thank you
     int main(void) {return 0;}
 ```
 ````