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.

user/downloading-installing-upgrading/installation-guide.md

@@ -462,7 +462,7 @@ the other [How-To Guides](/doc/#how-to-guides), and learn about
   useful and user friendly. We urge you to read it! It may very well contain
   the answers to your questions. (Since the documentation is a community
   effort, we'd also greatly appreciate your help in
-  [improving](/doc/doc-guidelines/) it!)
+  [improving](/doc/how-to-edit-the-documentation/) it!)
 
 * If issues arise during installation, see the [Installation
   Troubleshooting](/doc/installation-troubleshooting) guide.