docs: add a section about advisories and vulnerabilities

docs/book/modules/concepts/nav.adoc

@@ -1 +1,2 @@
 * xref:concepts:index.adoc[Concepts]
+** xref:concepts:a_v.adoc[Advisories & vulnerabilities]

docs/book/modules/concepts/pages/a_v.adoc

@@ -0,0 +1,11 @@
+= Advisories & vulnerabilities
+
+Trustify learns about xref:index.adoc#vulnerability[Vulnerabilities] by ingesting advisories. During the ingestion
+process, Trustify extracts and aggregates vulnerability information, grouped by their vulnerability identifier.
+
+Advisories can contain multiple vulnerabilities and can scope the application of statements the advisories make to
+certain packages. This means that Trustify has an aggregated set of information for a vulnerability, where information
+from the Common Vulnerabilities and Exposures (CVE) project supersedes information from more specific advisories.
+
+Trustify also has "vulnerabilities belonging to an advisory", which contain specific vulnerability information,
+provided by that advisory.

docs/book/modules/concepts/pages/index.adoc

@@ -2,9 +2,8 @@
 
 The following sections explain a few concepts of Trustify.
 
-== Entities
-
-=== Vulnerability
+[#vulnerability]
+== Vulnerability
 
 A vulnerability is mostly, primarily a *name* that is used to ensure all advisories are discussing the same thing.
 Generally, to this point, most vulnerabilities come from the CVE Project, with the format of `CVE-2024-1234`.
@@ -13,7 +12,7 @@ Within the database, generally a vulnerability is added as a side effect of an a
 
 A *CVE Record* from NIST/NVD is a low-value advisory that is generally the first discovered advisory that mentions a vulnerability.
 
-=== Advisory
+== Advisory
 
 An advisory is an opinion about a vulnerability.
 
@@ -27,7 +26,7 @@ This may be simply in reference to the vulnerability *as it exists in source-cod
 Other, more-involved stakeholders (product vendors, upstream project owners) may issue *additional* advisories.
 These opinions may be in reference to *concrete* shipped products, contextualized to how the vulnerable code is *actually used*.
 
-=== SBOM
+== SBOM
 
 An SBOM is a source-of-someone's-truth about "what's inside it?", so
 everything in our DB is ultimately sourced from some
@@ -39,7 +38,7 @@ A1 + A97". So an SBOM is the entity to track the origin of the
 supposed "evidence" of assertional statements about products... about
 packages... about vulnerabilities...
 
-=== Package
+== Package
 
 A package is an atomic artifact or component.
 Packages may be addressed using pURLs.
@@ -48,15 +47,15 @@ A package may certainly contain other packages (e.g. shading one Java jar into a
 A package may also be the sole member of a Product (`UBI-8.0.13-x86.oci` may be the singular package within the "UBI 8.0.13-x86" product).
 A package is one step more abstract than an *artifact*.
 
-==== pURL
+=== pURL
 
 Package URLs (pURLs) are possibly ambiguous names applied to packages.
 A simple pURL such as `pkg:maven/org.apache/[email protected]` may or may not refer to a unique artifact.
 With additional qualifiers, it is possible to produce a URI that asserts uniqueness, such as `pkg:maven/org.apache/[email protected]?repository_url=repo.jboss.com`.
 Without additional qualifiers, the implicit aspects (such as `repository_url`) must be taken into account.
 For instance, an unqualified `pkg:maven` pURL *implies* "the jar from Maven Central, and none other".
 
-=== Product
+== Product
 
 A product is a *named collection of 1 or more packages* for a concrete shippable thing.
 
@@ -68,7 +67,7 @@ NOTE: Given Red Hat ProdSec definitions, grouping of Products may need to occur
 `RHEL8` may be a *product stream*.
 `RHEL 8.2.03 PowerPC` may be a concrete *product* distinct from `RHEL 8.2.03 AArch64`.
 
-==== CPE
+=== CPE
 
 A CPE is a "Common Product Enumeration" from the NIST organization.
 CPEs are self-assigned but registered occasionally with NIST.
@@ -78,7 +77,7 @@ For instance, "All versions of RHEL 8.2.013, regardless of platform", or if more
 
 NOTE: CPEs are somewhat contentious, and used enough for us to not ignore, but not used enough to be a pivotal definition of "product" for any users of Trustify.
 
-=== Artifact
+== Artifact
 
 For a given *package*, there may be zero or more instances of that package.
 Given `log4j-1.2.3.jar`, seventeen different people could compile the same source with the same arguments, and still end