Reorganize doc guidelines into multiple pages; update style guide

The existing doc guidelines page attempts to combine too many different
topics at once and includes information that does not pertain directly
to the documentation. This reorganization is intended to make each type
of information easier to find. For example, some have found it difficult
to find the documentation style guidelines (see, e.g.,
QubesOS/qubes-issues#6701#issuecomment-875875610). This reorganization
allows us to assign more specific titles to each page.

General changes:
- Create new page for contribution instructions
- Create new page for website style guide
- Create new page for continuous integration
- Rename existing "style guide" to "visual style guide" in order to
  avoid ambiguity with new doc and website style guides
- Retain existing page solely for doc style guide
- Update page names and permalinks
- Update existing links
- Improve language

Doc style guide changes:
- Add section on using sentence case in headings
  (see QubesOS/qubes-issues#6756 and #1173)
- Improve section organization
- Clarify language

In order to better preserve the Git history of each file, file renames
will be handled in a separate commit.

introduction/faq.md

@@ -267,7 +267,9 @@ Please see the coreboot website / their IRC channel for further information.
 
 ### How should I report documentation issues?
 
-Please see the [documentation guidelines](/doc/doc-guidelines).
+If you can fix the problem yourself, please see [how to edit the
+documentation](/doc/how-to-edit-the-documentation). If not, please see [issue
+tracking](/doc/issue-tracking).
 
 ### Will Qubes seek to get certified under the GNU Free System Distribution Guidelines (GNU FSDG)?
 
@@ -279,7 +281,7 @@ This website is hosted on [GitHub Pages](https://pages.github.com/) ([why?](#why
 Therefore, it is largely outside of our control.
 We don't consider this a problem, however, since we explicitly [distrust the infrastructure](#what-does-it-mean-to-distrust-the-infrastructure).
 For this reason, we don't think that anyone should place undue trust in the live version of this site on the Web.
-Instead, if you want to obtain your own trustworthy copy of this website in a secure way, you should clone our [website repo](https://github.com/QubesOS/qubesos.github.io), [verify the PGP signatures on the commits and/or tags](/security/verifying-signatures/#how-to-verify-qubes-repos) signed by the [doc-signing keys](https://github.com/QubesOS/qubes-secpack/tree/master/keys/doc-signing) (which indicates that the content has undergone review per our [documentation guidelines](/doc/doc-guidelines)), then either [render the site on your local machine](https://github.com/QubesOS/qubesos.github.io/blob/master/README.md#instructions) or simply read the source, the vast majority of which was [intentionally written in Markdown so as to be readable as plain text for this very reason](/doc/doc-guidelines/#markdown-conventions).
+Instead, if you want to obtain your own trustworthy copy of this website in a secure way, you should clone our [website repo](https://github.com/QubesOS/qubesos.github.io), [verify the PGP signatures on the commits and/or tags](/security/verifying-signatures/#how-to-verify-qubes-repos) signed by the [doc-signing keys](https://github.com/QubesOS/qubes-secpack/tree/master/keys/doc-signing) (which indicates that the content has undergone [review](/doc/how-to-edit-the-documentation/#security)), then either [render the site on your local machine](https://github.com/QubesOS/qubesos.github.io/blob/master/README.md#instructions) or simply read the source, the vast majority of which was [intentionally written in Markdown so as to be readable as plain text for this very reason](/doc/documentation-style-guide/#markdown-conventions).
 We've gone to special effort to set all of this up so that no one has to trust the infrastructure and so that the contents of this website are maximally available and accessible.
 
 ### What does it mean to "distrust the infrastructure"?