Niggly User Guide Chap 2/3 updates [skip appveyor]

[mwichmann]

Including:

• New standard header
• Markup, including use provided for SConstruct files instead of (renders the same, ATM).
• Add a Fortran Hello, world for fun.
• Don't use "we" or "our" (generic documentation advice, supposedly presents an air of superiority)
• Add xml id to sections, to make permalinks (without, tools generate a numeric ID which may change over time, like: https://scons.org/doc/production/HTML/scons-user.html#idp105549032602952)

This is a doc-only change.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt (and read the README.rst)
• I have updated the appropriate documentation