Documentation for hook_civicrm_alterEntityRefParams

Move tokens docs into User Guide

related:
https://github.com/civicrm/civicrm-user-guide/pull/232

Resolve #316

Document API lookups by username

mkdocs.yml - Simplify menu headings

Per discussion of https://github.com/civicrm/civicrm-dev-docs/pull/434

When scanning the navbar, there are several words which (IMHO) aren't adding
much value.  This simplifies and should make it easier to navigate.

change path in Entity creation w/flow

framework/filesystem.md - Cleanup prose. Add table of `civicrm.files` subdirs.

documentation/style-guide.md - Mention bash portability tip

Add "Review Standards"

If a reader has customized their interactive shell, their system could have
small discrepancies in shell notation.  It shouldn't matter 95% of the time,
and it's not critical to support those folks perfectly.  But it is nice.  So
let's just share the link to heighten awareness.

(Example: I usually do `export VAR=value`...  but in future docs, I'll try
to use `VAR=value; export VAR`.)

This is following-up on some comments from @ginkgomzd who found the
`bash-hackers.org` site and shared it on https://github.com/civicrm/civicrm-buildkit/pull/365

 * Simplify titles under "Common standards". (Use a noun-phrase instead of an imperative statement.)
 * Combine `r-ext`, `r-core` into r-technical. Add more details. Reference `#gotchas`.
 * Rename `r-read` to `r-code`
 * Rename `rg-schema` to `rg-upgrade`. Add more details.

Add warning about TRUNCATE TABLE to PHPUnit Tests doc.

standards/php.md - Add a TLDR example

To see actual examples of the coding style, you need to skim multiple blurbs
and click multiple links, and the eventual page shows very isolated
examples.

This revision adds a brief example so that a new contributor can get a
general sense of the layout immediately.

(Note: It's not possible for a brief example to show every edge-case, and we
shouldn't try -- that's why the commit also emphasizes the link into the
relevant examples from d.o docs.)

Specifically:

 * Alphabetize
 * Add general description

Add doc for WordPress shortcodes in extensions

typo

Asset builder example

ref:
https://github.com/civicrm/civicrm-wordpress/pull/112

I'm not sure if this has only come about because of my ineptitude (i.e. whether an Entity would always have the same name as the extension), but I found that I couldn't find the files referred to because my Entity and my Extension had different names.

Would that be bad practice on my part? (it would be good to know as I'm very new to coding)

Either way, I think this may be more accurate... rest is up to you.

Cheers,
M.