tutorial: support, and start using, links to other sections

This patch adds supports for links to other sections in the tutorial.
These links look like [Lifetime management], i.e., the title of the
section (can the title of a chapter or of a smaller subsection) in
square brackets.

The links work on both pdf and HTML outputs (generated by the ninja
targets doc/tutorial.pdf and doc/tutorial.html, respectively).

Signed-off-by: Nadav Har'El <[email protected]>
Message-Id: <[email protected]>

configure.py

@@ -1005,10 +1005,10 @@ def have_hwloc():
             description = CSCOPE
         build cscope: cscope
         rule md2html
-            command = pandoc --self-contained --smart --toc -c doc/template.css -V documentclass=report --chapters --number-sections -f markdown_github+pandoc_title_block --highlight-style tango $in -o $out
+            command = pandoc --self-contained --smart --toc -c doc/template.css -V documentclass=report --chapters --number-sections -f markdown_github+pandoc_title_block+implicit_header_references --highlight-style tango $in -o $out
             description = PANDOC $out
         rule md2pdf
-            command = pandoc -f markdown_github+pandoc_title_block --highlight-style tango --template=doc/template.tex $in -o $out
+            command = pandoc -f markdown_github+pandoc_title_block+implicit_header_references --highlight-style tango --template=doc/template.tex $in -o $out
             description = PANDOC $out
         rule htmlsplit
             command = cd doc; ./htmlsplit.py

doc/tutorial.md

@@ -1523,7 +1523,7 @@ The most basic building block for writing promises is the **promise object**, an
 CONTINUE HERE. write an example, e.g., something which writes a message every second, and after 10 messages, completes the future.
 
 # Seastar::thread
-Seastar's programming model, using futures and continuations, is very powerful and efficient.  However, as we've already seen in examples above, it is also relatively verbose: Every time that we need to wait before proceeding with a computation, we need to write another continuation. We also need to worry about passing the data between the different continuations (using techniques like those described in the "Lifetime management" section). Simple flow-control constructs such as loops also become more involved using continuations. For example, consider this simple classical synchronous code:
+Seastar's programming model, using futures and continuations, is very powerful and efficient.  However, as we've already seen in examples above, it is also relatively verbose: Every time that we need to wait before proceeding with a computation, we need to write another continuation. We also need to worry about passing the data between the different continuations (using techniques like those described in the [Lifetime management] section). Simple flow-control constructs such as loops also become more involved using continuations. For example, consider this simple classical synchronous code:
 ```cpp
     std::cout << "Hi.\n";
     for (int i = 1; i < 4; i++) {