Document generation and fixes default run sg

[j0yu]

Changes from v1.0.0+wwfx.0.2.0 to v1.0.0+wwfx.0.3.0

Functional improvements and fixes from testing tk-doc-generator (locally using preview_docs.sh and on travis.ci) with tk-katana on the ci-docs branch/PR. See also tk-katana's gh-pages

• Dockerfile:

  • Moved Gem* from /app to /usr/local/src.
    • Keeps /app clean for docker run.
  • Added default ENTRYPOINT to run build_docs.sh.
  • Added tk-core as Sphinx conf.py requires it.
  • Optimised yum install packages, Ruby curl/untar.
  • Using nproc, replacing hard-coded jobs count.

• preview_docs.sh:

  • Support -v instead of --mount for docker run when using older docker version shipped with CentOS 7.
  • Will not re-build Docker image if tk-doc-generator image exists.
    • Force rebuild using preview_docs.sh --rebuild.
  • Can now be run anywhere, including within tk-doc-generator

• build_docs.sh:

  • Fixed configurations using absolute paths.
  • Fixed inaccurate echo statements

• Gemfile*: Using WWFX just-the-docs with new external links test

  • May want to change these Gemfile* if the PR gets merged