sponsors: link to sponsor requirements doc

Also, fix a minor HTML tidy issue.

_sponsors.html

@@ -51,6 +51,7 @@
 TITLE(Sponsors)
 <div class="relatedbox">
 <b>Related:</b>
+<br><a href="https://github.com/curl/curl/blob/master/docs/SPONSORS.md">Sponsor requirements</a>
 <br><a href="/donation.html">Donate</a>
 <br><a href="/docs/help-us.html">Help us</a>
 <br><a href="/docs/">Online Docs</a>
@@ -99,7 +100,7 @@
 
 <div style="clear: both;"></div>
 SUBTITLE(Silver sponsors)
-<!---- Silver start ----->
+<!-- Silver start -->
 <div class="silver"><p> <a class="x" href="https://icons8.com/" rel="sponsored"><img src="pix/silver/icons8.svg" alt="icons8"></a></div>
 <div class="silver"><p> <a class="x" href="https://www.minitool.com" rel="sponsored"><img src="pix/silver/minitool-2.svg" alt="minitool-2"></a></div>
 <div class="silver"><p> <a class="x" href="https://www.airbnb.com/" rel="sponsored"><img src="pix/silver/airbnb.svg" alt="airbnb"></a></div>
@@ -123,7 +124,7 @@
  <div class="silver"><p> <a class="x" href="https://casino.xyz/" rel="sponsored"><img src="pix/silver/casinoxyz.png" alt="casino.xyz"></a></div>
 <!-- Silver for curl on github -->
  <div class="silver"><p> <a class="x" href="https://www.celebian.com/" rel="sponsored"><img src="pix/silver/celebian.png" alt="celebian"></a></div>
-<!---- Silver end ----->
+<!-- Silver end -->
 <div style="clear: both; max-width: 40em; margin-left: auto; margin-right: auto;">
 SUBTITLE(Join in and Sponsor!)
 <p>