Inline introduction.adoc

Having it in a separate file confuses VSCode's Asciidoc support a bit because the heading sequence isn't correct - it goes H1->H3. I think it's also less confusing for humans this way, because only the first part of the introduction was in the introduction.adoc, and it hardly seems worth having a separate file for one paragraph.

Author: Timmmm

riscv-plic.adoc

@@ -54,8 +54,15 @@ include::changelog.adoc[]
 include::licensing.adoc[]
 // Contributors of this document
 include::contributor.adoc[]
-// introduction of this document
-include::introduction.adoc[]
+
+== Introduction
+
+This document specifies the Platform-Level Interrupt Controller (PLIC). PLIC is
+designed to work in the context of RISC-V systems and allows various
+device interrupts to be multiplexed onto the external interrupt lines of one or
+more Hart contexts, with hardware support for interrupt priorities. PLIC supports
+up-to 1023 interrupts (0 is reserved) and 15872 contexts. The actual number of
+interrupts and context depends on the PLIC implementation.
 
 === Interrupt Targets and Hart Contexts