Step 6 on 2093: userguide on entry points now available

Correct some of the behaviors indicated in new entry points docs. Switch to imperative voice. Limit to declarative config (setup.cfg) examples.

Coverage
1. console_script
2. plugin support
3. optional dependencies

The `decalrative config`_ link present higher in the document is also
unresolved, I'm not quite sure what the intended target was.

Step 5 on 2093: Package discovery and namespace package userguide now ready

userguide/pkg_discovery.txt now covers find_package,
find_namespace package and legacy use of namespace
package creation in both setup.py and setup.cfg style

Step 4: A PEP517-styled quickstart user guide and room for collaboration

Incorporate PR 1765's keywords.txt

before: pep517 build (incorrect)
now: python -m pep517.build (correct)

created directory setuptools/docs/references and place
keywords.txt from PR1765 in it en bloc

added a few sections (WIP) to make it more complete

It is now declarative and covers the following aspects of packaging:

- installation
- basic use
- package discovery
- entry points
- dependencies
- data files

Each section comprises a brief demonstration of the functionality
and provide a link to more advanced explanation

WIP

to make it pep517-compatible and declarative

doc overhaul step 3: update userguide

these should be properly grouped but for now I'm just
leaving them here

those that don't need to put into a dedicated file
are now inside the miscellaneous.txt