manuals: replace hyphens with em dashes

Fix some hyphens being improperly used as em dashes.
See https://www.grammarly.com/blog/hyphens-and-dashes/

Using em dashes may also allow Sphinx to hyphenate
and break lines in the best way.

Note that the first character after an em dash not
supposed to be capitalized, unless a specific
rule applies, typically when what follows is a proper noun.

Fix a few misuses of parentheses in following text.

(From yocto-docs rev: a0d93ea1ddfdfbcde8dac3aa328307be778f9e3c)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/structure.rst

@@ -213,8 +213,8 @@ These files are standard top-level files.
 
 .. _structure-build:
 
-The Build Directory - ``build/``
-================================
+The Build Directory --- ``build/``
+==================================
 
 The OpenEmbedded build system creates the :term:`Build Directory`
 when you run the build environment setup
@@ -589,7 +589,7 @@ install" places its output that is then split into sub-packages within
 ``build/tmp/work/tunearch/recipename/version/``
 -----------------------------------------------
 
-The recipe work directory - ``${WORKDIR}``.
+The recipe work directory --- ``${WORKDIR}``.
 
 As described earlier in the
 ":ref:`structure-build-tmp-sysroots`" section,
@@ -654,8 +654,8 @@ recipes. In practice, this is only used for ``gcc`` and its variants
 
 .. _structure-meta:
 
-The Metadata - ``meta/``
-========================
+The Metadata --- ``meta/``
+==========================
 
 As mentioned previously, :term:`Metadata` is the core of the
 Yocto Project. Metadata has several important subdivisions: