fix more broken markdown links

We fix more markdown links in markdown files that refer to files
internally to the web site.

Added tmp tools to find and print various markdown links:

* `tmp/md-links.sh` - print all markdown links
* `tmp/md-token-links.sh` - print all markdown links that start with a **%%TOKEN%%**
* `tmp/md-local-links.sh` - print all markdown links to a local file
* `tmp/md-link-paths.sh` - print local file paths referenced in markdown links

Fixed a number of markdown formatting problems with `thanks-for-help.md`.
Fixed markdown links to files in `thanks-for-help.md`.  Improved
heading levels in `thanks-for-help.md`, and made entry sections consistent.

Changed all markdown links that end in "))" to end in a better way.

Explained in `markdown.md` why we are avoiding markdown links that
end in "))".

Ran `make www` to perform the above.

TODO: Fix stuff we broke by mistake doing the above.

TODO: Fix more broken markdown links.

markdown.md

@@ -12,6 +12,8 @@ See the [markdown syntax](https://www.markdownguide.org/basic-syntax) guide.
 See also [CommonMark Spec](https://spec.commonmark.org/current/).
 
 Nevertheless, the IOCCC does have certain practices that we ask authors to follow.
+Some these relate to use of markdown directly, others relate to injecting HTML
+into the markdown file.
 
 In particular there are things we ask people to please do **NOT** use markdown files:
 
@@ -195,3 +197,87 @@ Or better and easier still, use an inline markdown code block:
 ```
     `inline markdown code block is easier and much better`
 ```
+
+
+## Do not add trailing slash on void HTML elements
+
+Please do not use trailing slash on [void HTML
+elements](https://github.com/validator/validator/wiki/Markup-»-Void-elements).
+
+See also this note on [trailing slashes in void-element start
+tags](https://github.com/validator/validator/wiki/Markup-»-Void-elements#trailing-slashes-in-void-element-start-tags-do-not-mark-the-start-tags-as-self-closing).
+
+The trailing slash on void HTML elements has no effect and interacts badly with unquoted attribute values.
+
+For example, please do not use:
+
+```
+    <br/>	<=== no thank you
+```
+
+Instead use just:
+
+```
+    <br>
+```
+
+And for example, please do not use:
+
+```
+    <hr/>	<=== no thank you
+```
+
+Instead use just:
+
+```
+    <hr>
+```
+
+And for example, please do not use:
+
+```
+    <img src="1984-anonymous-tattoo.jpg"
+     alt="image of a tattoo of the 1984 anonymous C code"
+     width=600 height=401 />					<=== no thank you
+```
+
+Instead use just:
+
+```
+    <img src="1984-anonymous-tattoo.jpg"
+     alt="image of a tattoo of the 1984 anonymous C code"
+     width=600 height=401>
+```
+
+etc.
+
+## Do not end markdown links in ))
+
+Please do not end a markdown links with a double closed parenthesis "))".
+
+Markdown links that end in "))" complicate parsing and sometimes lead
+to incorrect URLs or file paths.
+
+Instead of:
+
+```
+[some text](https://example.com/foo_(bar))	<=== no thank you
+```
+
+Use:
+
+```
+[some text](https://example.com/foo_&#x28;bar&#x29;)
+```
+
+Instead of:
+
+```
+This thing ([some text](some/path)) is not ideal.	<=== no thank you
+``
+
+Use:
+
+```
+This thing, [some text](some/path), is better.
+```