Fix HTML document titles

Summary:
The document titles should be explicitly passed, otherwise pandoc generates a warning and tries to guess

Before:
```
folly/folly/docs on  master
❯ make Format.html
/usr/bin/pandoc -s -H style.css -f markdown -t html --toc -o Format.html Format.md
[WARNING] This document format requires a nonempty <title> element.
  Defaulting to 'Format' as the title.
  To specify a title, use 'title' in metadata or --metadata title="...".

folly/folly/docs on  master
❯ make index.html
/usr/bin/pandoc -s -H style.css -f markdown -t html --toc -o index.html *.md
[WARNING] This document format requires a nonempty <title> element.
  Defaulting to 'AtomicHashMap' as the title.
  To specify a title, use 'title' in metadata or --metadata title="...".
```

After:
```
folly/folly/docs on  fix-docs [!]
❯ make Format.html
/usr/bin/pandoc -s -H style.css -f markdown -t html --toc -o Format.html Format.md --metadata title="folly/Format.h"

folly/folly/docs on  fix-docs [!]
❯ make index.html
/usr/bin/pandoc -s -H style.css -f markdown -t html --toc -o index.html *.md --metadata title="Folly API Documentation"
```

Reviewed By: danobi

Differential Revision: D24546374

fbshipit-source-id: 818e6ce39f361695a5f94c7662953062ea2f2868

folly/docs/Makefile

@@ -20,13 +20,13 @@ html: $(HTML)
 	$(PANDOC) -f markdown -o $*.pdf $*.md
 
 %.html: %.md style.css
-	$(PANDOC) $(PANDOCARGS) -H style.css -f markdown -t html --toc -o $*.html $*.md
+	$(PANDOC) $(PANDOCARGS) -H style.css -f markdown -t html --toc -o $*.html $*.md --metadata title="$(shell head -n1 $*.md | sed -e 's|`||g')"
 
 docs.md: $(SOURCES) style.css
 	$(PANDOC) $(PANDOCARGS) -H style.css -f markdown -t markdown --toc -o $@ *.md
 
 index.html: $(SOURCES) style.css
-	$(PANDOC) $(PANDOCARGS) -H style.css -f markdown -t html --toc -o $@ *.md
+	$(PANDOC) $(PANDOCARGS) -H style.css -f markdown -t html --toc -o $@ *.md --metadata title="Folly API Documentation"
 
 
 clean: