How-To Update for Docs (building on #816)

[pzrq]

This PR builds on #819, specifically it:

• resolves the merge conflicts with master
• fixes some typos
• adds some links, especially to the builtins guide
• migrates the builtins guide from list() (which I think is a more advanced-level guide, and also covered better in types) to a more intermediate-level guide with the max() function. I'm not wedded to max(), its errors just seemed a little more interesting than abs()

Many of the changes above were suggested as feedback on #816, so I have done what I can to move this forward.

PR Checklist:

• [x] All new features have been tested
• [x] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct

[pzrq]

Aside: I see a warning when running make html which appears to be fixed upstream in https://github.com/readthedocs/sphinx_rtd_theme/pull/728

(venv) pzrq@mozzie:~/Projects/beeware/batavia/docs$ make html
sphinx-build -b html -W -d _build/doctrees   . _build/html
Running Sphinx v2.1.2
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 8 source files that are out of date
updating environment: [] 0 added, 8 changed, 0 removed
reading sources... [100%] how-to/types                                          
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] index                                                  
generating indices... genindex
writing additional pages... search/Users/pzrq/Projects/beeware/venv/lib/python3.7/site-packages/sphinx_rtd_theme/search.html:20: RemovedInSphinx30Warning: To modify script_files in the theme is deprecated. Please insert a <script> tag directly in your theme instead.
  {{ super() }}

copying static files... done
copying extra files... done
dumping search index in English (code: en) ... done
dumping object inventory... done
build succeeded.

The HTML pages are in _build/html.
Copying tabs assets

Build finished. The HTML pages are in _build/html.