Start porting documentation from wiki to website

Port a few pages over from the wiki to the website.

I’ve tried to make One True Source for instructions on building GS2, which is now on the website. The README at the top of the repo contains a brief example. This could be larger, but I didn’t want to repeat too much.

Added a page on how to write documentation. This is possibly missing some steps. A partially rendered version is here: https://bitbucket.org/gyrokinetics/gs2/src/9cd1702c2d18b640766713bcce5b4d1190253501/docs/page/developer_manual/writing_documentation.md?at=micro%2Fporting-docs

but the version on the website will have working links to other pages and code docs.

• Docs: Port introduction page from wiki to website docs

  Note: EFIT and TOQ links are broken. Better domain might be fusion.gat.com, but this is password-protected

• Docs: Migrate intro page from wiki; consolidate build instructions

• Docs: Port some basic pages from wiki

  • coordinates

  • normalisation

  • results and diagnostics

  • timestepping

  Still some broken links

• Docs: Fix some links and code formatting in user manual

• Docs: Add documentation on writing documentation

  Fixes #109

• Docs: Move out of source documentation to docs/page

  FORD always puts the output in docs/html/page, so let's be consistent

• Docs: Make path into manual friendlier to discover