Edit README.ftl.md and SIDEBAR.md

-Add build_an_executable_jar_subhead to README.ftl.md
-Revise Summary per language for other summaries
-Revise SIDEBAR.md per language for other Related Resources
-Revise "Understanding" to "Concepts and Technologies" per others

-Avoid excessive italics in README.ftl.md Looks jarring.
-In Summary section, suggest adding a link to Related Resources
(SIDEBAR.md) so reader doesn't have to scroll all the way back up page.
You don't want to put links in Summary because it's redundant and
Summary is about summing up current guide.
-Make Related Resources a main head, edit intro.

Making 'Related Rerources' title smaller in the sidebar

 - Add link to actual GSG
 - Add Tutorials section
 - Clean up Understanding section

Note, this was processed using the following command line:

    fpp --input=README.ftl.md --output=README.md

and output suffers from the bug detailed at:

    https://www.pivotaltracker.com/story/show/52221207

In *Create a main class* moved first paragraph of text to beginning of
section, before code.>>>>>>> upstream/master

- Change admonition style to use blockquote syntax

 - Revise all exposition
 - Remove gradle artifacts and instructions for now
 - Eliminate @Configuration class
 - Convert tabs to spaces in .java and .xml files
 - Preserve empty directory structure in 'initial'

[http://localhost]() syntax used previously doesn't actually work. Link
is created, but link is back to self.

Per a [hot tip][1], now using <http://localhost> syntax, which works
perfectly.

[1]: https://github.com/vmg/sundown/issues/132#issuecomment-8774103

 - http://foo.com will auto-link
 - http://foo.com:8080 will too
 - http://localhost won't
 - http://localhost:8080 won't either

Workaround is to wrap in link syntax, e.g.

 - [http://localhost:8080]()

Per http://github.github.com/github-flavored-markdown/

Introduce mocked-up macro syntax with the goal of eliminating
duplication of common sections, introductions, disclaimers, etc.

See gs-macros.md[1] for expanded values.

[1]: https://github.com/springframework-meta/springframework.org/tree/master/doc/gs-macros.md

 - Standardize capitalization across title and subsection headers
 - Standardize grammar of title and subsection headers
 - Standardize link format

Also now wrap Maven/Gradle sections in <span> tags with 'maven' and
'gradle' class values to allow us to apply CSS to these to hide or show
based on value of Maven / Gradle toggle control in UI. Ideally this
should be a <div>, but by default Markdown syntax is not supported (i.e.
parsed) within divs. See
http://daringfireball.net/projects/markdown/syntax#html for details.