docs: fix xref to toolchain docs from getting starting (#2899)

The "toolchains" xref is ambiguous. Create a unique header for the
toolchain configuration
section and link to that name instead.

docs/getting-started.md

diff --git a/docs/getting-started.md b/docs/getting-started.md
index 9697166..60d5d5e 100644
--- a/docs/getting-started.md
+++ b/docs/getting-started.md
@@ -7,7 +7,7 @@
 It assumes you have a `requirements.txt` file with your PyPI dependencies.
 
 For more details information about configuring `rules_python`, see:
-* [Configuring the runtime](toolchains)
+* [Configuring the runtime](configuring-toolchains)
 * [Configuring third party dependencies (pip/pypi)](pypi-dependencies)
 * [API docs](api/index)
 

docs/toolchains.md

diff --git a/docs/toolchains.md b/docs/toolchains.md
index 121b398..a2a2b5b 100644
--- a/docs/toolchains.md
+++ b/docs/toolchains.md
@@ -1,6 +1,7 @@
 :::{default-domain} bzl
 :::
 
+(configuring-toolchains)=
 # Configuring Python toolchains and runtimes
 
 This documents how to configure the Python toolchain and runtimes for different