In Doxygen 1.9.7, the URL generation logic changed such that input
files in directories (i.e. /docs/Hooks.md) are published with the
slug `md_docs_2Hooks.html` instead of `md_docs_Hooks.html`.
This was done to improve an edge case where if two conflicting
files existed (e.g. docs_Foo.md and docs/Foo.md) both can now get
their own stable URL in a determimistic way (e.g. as opposed to
a-z sorting and appending _2 only if a conflict exists). But this
made other more common URLs no longer stable. Work around this by
setting an explict permalink for each markdown file.
Ref https://github.com/doxygen/doxygen/issues/10721.
Change-Id: Ifeb03602452c1148bd372555bebac9922c583ac2
