[NFC][analyzer][docs] Migrate 'annotations.html' to RST

This commit migrates the contents of 'annotations.html' in the old
HTML-based documentation of the Clang static analyzer to the new
RST-based documentation.

During this conversion I reordered the sections of this documentation
file by placing the section "Custom Assertion Handlers" as a subsection
of "Annotations to Enhance Generic Checks". (The primary motivation was
that Sphinx complained about inconsistent section levels; with this
change I preserved that sections describing individual annotations are
all on the same level.)

Apart from this change and the format conversion, I didn't review,
validate or edit the contents of this documentation file because I think
it would be better to place any additional changes in separate commits.

Author: NagyDonat

clang/www/analyzer/images/example_attribute_nonnull.png renamed to clang/docs/analyzer/images/example_attribute_nonnull.png

@@ -12,4 +12,5 @@ Contents:
    user-docs/FilingBugs
    user-docs/CrossTranslationUnit
    user-docs/TaintAnalysisConfiguration
+   user-docs/Annotations
    user-docs/FAQ