Standardize site-wide capitalization

- Use **Sentence casing** for all titles and section headers
- Words after a **":"** remain capitalized
- A few section name rewrites for shorter length, better readability

Source: https://developers.google.com/style/capitalization#capitalization-in-titles-and-headings_1

Serves https://github.com/bazelbuild/bazel/issues/10794

Closes #10827.

PiperOrigin-RevId: 296278672
diff --git a/site/docs/user-manual.html b/site/docs/user-manual.html
index db6fabe..fcc09f0 100644
--- a/site/docs/user-manual.html
+++ b/site/docs/user-manual.html
@@ -1,10 +1,10 @@
 ---
 layout: documentation
-title: User Manual
+title: User manual
 ---
-<h1>Commands and Options</h1>
+<h1>Commands and options</h1>
 
-<h2 id='target-patterns'>Target Syntax</h2>
+<h2 id='target-patterns'>Target syntax</h2>
 
 Some commands, like <code>build</code> or <code>test</code>, can operate
 on a list of targets. They use a syntax more flexible than labels, which is