[SYCL][DOC] set myst implicit targets setting

Doc build is getting error after upgrading to latest sphinx.

2023-10-26T21:46:24.1306605Z Warning, treated as error:
2023-10-26T21:46:24.1308815Z
/home/runner/work/llvm/llvm/repo/sycl/doc/EnvironmentVariables.md::'myst'
cross-reference target not found: 'oneapi_device_selector'
[myst.xref_missing]

sycl/doc/conf.py

@@ -36,6 +36,9 @@
     'myst_parser'
 ]
 
+# Implicit targets for cross reference
+myst_heading_anchors = 4
+
 # The name of the Pygments (syntax highlighting) style to use.
 pygments_style = 'friendly'