convert SGML guide to RST

convert SGML guide to RST

the rationale behind switching away from SGML/Docbook is the following:

 * SGML is hard to edit for humans
 * the output is not much prettier
 * the toolchain is not well supported and missing from the build
 * the build is not hooked into anywhere, no automation

the reason why RST was chosen:

 * it allows for a strong structure like Docbook
 * the theme from Read The Docs is pretty
 * it also supports mobile devices
 * sphinx can easily output to PDF and ePUB formats
 * RST is plaintext that can be easily edited and diff'd
 * RST can be automatically built by ReadTheDocs and the toolchain is
   readily available
 * the output is also parsed by Github so documentation can be read
   straight from GH

the reason why Markdown was not chosen:

 * the current strong structure would be hard to replicate
 * markdown is not standardized and output varies according to the
   implementation

the docs were converted with Pandoc, using the following commands:

    mkdir oper-guide
    for source in sgml/oper-guide/*.sgml; do
        pandoc --toc -s -f docbook -t rst $source -o oper-guide/$(basename $source .sgml).rst
    done
    cd oper-guide
    sphinx-quickstart
    git add *.rst make.bat conf.py
    git add -f Makefile
    git rm -r ../sgml

[merged from d4214e9445d9f9d0f0ede3e09a9f81deee9: doc/sgml/oper-guide/config.sgml]