meson: disable doc builds by default

The only docs we build is the doxygen API and I reckon there's a near-zero
amount of users who actually want these built locally instead of just browsing
the online doc.

Let's not build this by default, dropping the requirement on doxygen and
speeding up the build by quite a fair bit.

Author: whot

.github/workflows/linux.yml

@@ -47,7 +47,7 @@ jobs:
       - name: Setup
         run: |
           # -gdwarf-4 - see https://github.com/llvm/llvm-project/issues/56550.
-          CFLAGS='-gdwarf-4' meson setup build -Denable-cool-uris=true
+          CFLAGS='-gdwarf-4' meson setup build -Denable-docs=true -Denable-cool-uris=true
         env:
           CC: ${{ matrix.compiler }}
       - name: Build

meson_options.txt

@@ -63,7 +63,7 @@ option(
 option(
     'enable-docs',
     type: 'boolean',
-    value: true,
+    value: false,
     description: 'Enable building the documentation',
 )
 option(