docs(tvix): use # for toplevel headings

See discussion in cl/11869. Let's use `#` consistently, we also use `##`
for deeper nested subheadings.

Change-Id: Ie25ebda708639fb617d456c275ae5a264fc4ce85
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11881
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Reviewed-by: toastal <toastal@posteo.net>

tvix/docs/src/eval/bindings.md

@@ -1,5 +1,4 @@
-Compilation of bindings
-=======================
+# Compilation of bindings
 
 Compilation of Nix bindings is one of the most mind-bending parts of Nix
 evaluation. The implementation of just the compilation is currently almost 1000

tvix/docs/src/eval/build-references.md

@@ -1,5 +1,4 @@
-Build references in derivations
-===============================
+# Build references in derivations
 
 This document describes how build references are calculated in Tvix. Build
 references are used to determine which store paths should be available to a

tvix/docs/src/eval/builtins.md

@@ -1,5 +1,4 @@
-Nix builtins
-============
+# Nix builtins
 
 Nix has a lot of built-in functions, some of which are accessible in
 the global scope, and some of which are only accessible through the

tvix/docs/src/eval/known-optimisation-potential.md

@@ -1,5 +1,4 @@
-Known Optimisation Potential
-============================
+# Known Optimisation Potential
 
 There are several areas of the Tvix evaluator code base where
 potentially large performance gains can be achieved through

tvix/docs/src/eval/recursive-attrs.md

@@ -1,5 +1,4 @@
-Recursive attribute sets
-========================
+# Recursive attribute sets
 
 The construction behaviour of recursive attribute sets is very
 specific, and a bit peculiar.

tvix/docs/src/eval/vm-loop.md

@@ -1,5 +1,4 @@
-tvix-eval VM loop
-=================
+# tvix-eval VM loop
 
 This document describes the new tvix-eval VM execution loop implemented in the
 chain focusing around cl/8104.

tvix/docs/src/store/api.md

@@ -1,5 +1,4 @@
-tvix-[ca]store API
-==============
+# tvix-[ca]store API
 
 This document outlines the design of the API exposed by tvix-castore and tvix-
 store, as well as other implementations of this store protocol.