docs: use Yul lexer to highlight Yul code segments.

Many commits squashed; turns out that with the combination of:

* Python v2.7,
* Sphinx v1.8.5, and
* Pygments v2.3.1

versions (old!) used in the CI, the only viable approach is:

* to use `code-block` directives with explicit language specification,
* to provide no file-local default using `highlight`, and
* to set language as `none` for grammar specifications.

Underlying are the following issues (again, for the old versions
listed above):

* Generic RST `code` doesn't work when language is `none`:

    Warning, treated as error:
    /root/project/docs/yul.rst:430:Cannot analyze code. No Pygments lexer found for "none".

  Additionally, it might be trying to fall back to the default
  (Solidity) if left unspecified.

* If a file-local default is specified using `highlight`, then
  `code-block` _must_ also provide a language:

    Warning, treated as error:
    /root/project/docs/yul.rst:185:Error in "code-block" directive:
    1 argument(s) required, 0 supplied.

* Sphinx seems to try the file-local default "yul" (specified with
  `highlight`) on `code` marked having language `json`:

    Warning, treated as error:
    /root/project/docs/yul.rst:130:Could not lex literal_block as "yul". Highlighting skipped.

* The only well-lexed highlighter for two of the three grammar
  specifications is `peg`, but it was added in Pygments v2.6.
  One of the grammars - in the "Formal Specification" section,
  the one after "We will use a destructuring notation for the
  AST nodes." - _must_ be left unhighlighted, with language set
  to `none`: all lexers do really poorly.

... And one should never, ever start treating warnings as mere
warnings, without having exhausted all other options.

Otherwise, it's a slippery slope, - and look where that brought
Gandhi: to being a strawman in every lousy argument to be had!..

docs/conf.py

@@ -17,7 +17,7 @@ import sys
 import os
 import re
 
-from pygments_lexer_solidity import SolidityLexer
+from pygments_lexer_solidity import SolidityLexer, YulLexer
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -27,6 +27,7 @@ def setup(sphinx):
     thisdir = os.path.dirname(os.path.realpath(__file__))
     sys.path.insert(0, thisdir + '/utils')
     sphinx.add_lexer('Solidity', SolidityLexer())
+    sphinx.add_lexer('Yul', YulLexer())
 
     sphinx.add_stylesheet('css/custom.css')