docs: add a new page with global flags and link to it from the command docs

In f544234 we removed the global flags from each command as it was
making each page very big and causing 1000s of lines of duplication in
the man page.

This change adds a new flags page with all the global flags on and
links each command page to it.

Fixes #3273

bin/make_manual.py

@@ -20,6 +20,7 @@ docs = [
     "filtering.md",
     "rc.md",
     "overview.md",
+    "flags.md",
 
     # Keep these alphabetical by full name
     "alias.md",