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.

developer/general/continuous-integration.md

@@ -0,0 +1,34 @@
+---
+lang: en
+layout: doc
+permalink: /doc/continuous-integration/
+title: Continuous Integration (CI)
+---
+
+This page explains the [continuous integration
+(CI)](https://en.wikipedia.org/wiki/Continuous_integration) infrastructure that
+the Qubes OS Project uses.
+
+## Website and documentation
+
+The following commands may be useful as a way to interact with our CI
+infrastructure on website
+([qubesos.github.io](https://github.com/QubesOS/qubesos.github.io)) and
+documentation ([qubes-doc](https://github.com/QubesOS/qubes-doc)) pull requests
+(PRs). Note that special permissions may be required to use some of these
+commands. These commands are generally issued by adding a comment to a PR
+containing only the command.
+
+- `PipelineRetry`: Attempts to run the entire build pipeline over again. This
+  can be useful if CI incorrectly uses a stale branch instead of testing the PR
+  as if it were merged into `master`.
+
+- `TestDeploy`: Deploys a test website, which is a live version of the Qubes
+  website as if this PR had been merged. This can be useful for previewing a PR
+  on a live public website. **Note:** You must wait for the site to finish
+  building before issuing this command, or else it will deploy an empty
+  website. To find the URL of the test website, look for text similar to "This
+  branch was successfully deployed" and a button named something like "View
+  deployment." Note that there are two different testing sites: `wwwtest` is
+  manually updated, whereas `wwwpreview` is managed by the `TestDeploy`
+  command.