Add basic support for tagging text in the spec

This adds two things:

1. Custom CSS to add a subtle dotted underline on hover to any element with an ID starting with `qx_` (an arbitrary prefix unlikely to be used by chance).
2. An example tag `qx_x0_is_zero`, tagging the text that specifies that x0 is hardwired to 0.

This just adds a single tag as an exmaple, but the intention is that such tags would be added throughout the spec, allowing coverage, test plans, tests, documentation, etc. to all link to specific parts of the spec.

The text of these tags can also be extracted via Asciidoc's docbook output format, enabling more use cases such as monitoring for spec changes.

Makefile

@@ -83,6 +83,7 @@ OPTIONS := --trace \
            -a mathematical-format=svg \
            -a pdf-fontsdir=docs-resources/fonts \
            -a pdf-theme=docs-resources/themes/riscv-pdf.yml \
+           -a docinfo=shared \
            $(XTRA_ADOC_OPTS) \
            -D build \
            --failure-level=ERROR

src/docinfo.html

@@ -0,0 +1,22 @@
+<style>
+/*
+Any ID starting with 'qx_' (arbitrarily chosen to be unlikely to
+be used by accident) is a snippet of specification text that has been
+specifically tagged to allow referencing it robustly, for use cases
+such as:
+
+* Linking coverage, tests plans, tests, assertions etc. to parts of
+  the spec that they cover.
+* Detecting when parts of the spec change. You can extract these
+  snippets via Asciidoc's docbook output and parsing the XML.
+* Linking documentation (e.g. the implementation defined parameter list)
+  to the spec.
+  
+This adds a subtle decoration to make it easier to find tagged text.
+*/
+[id^="qx_"]:hover {
+    text-decoration: underline;
+    text-decoration-style: dotted;
+    text-decoration-color: gray;
+}
+</style>

src/rv32.adoc

@@ -40,7 +40,8 @@ Most of the commentary for RV32I also applies to the RV64I base.
 
 <<gprs>> shows the unprivileged state for the base
 integer ISA. For RV32I, the 32 `x` registers are each 32 bits wide,
-i.e., `XLEN=32`. Register `x0` is hardwired with all bits equal to 0.
+i.e., `XLEN=32`.
+[#qx_x0_is_zero]#Register `x0` is hardwired with all bits equal to 0.#
 General purpose registers `x1-x31` hold values that various
 instructions interpret as a collection of Boolean values, or as two's
 complement signed binary integers or unsigned binary integers.