sagemathgh-40904: Fix missing `conf.py` files during docbuilding
    
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Fixes the issue mentioned in
sagemath#39030 (comment):

```
[tutorial ] Configuration error!
[tutorial ] Versions
[tutorial ] ========
[tutorial ] * Platform:         linux;
(Linux-6.15.7-arch1-1-x86_64-with-glibc2.42)
[tutorial ] * Python version:   3.13.7 (CPython)
[tutorial ] * Sphinx version:   8.2.3
[tutorial ] * Docutils version: 0.21.2
[tutorial ] * Jinja2 version:   3.1.6
[tutorial ] * Pygments version: 2.19.2
[tutorial ] Last Messages
[tutorial ] =============
[tutorial ] None.
[tutorial ] Loaded Extensions
[tutorial ] =================
[tutorial ] None.
[tutorial ] Traceback
[tutorial ] =========
[tutorial ]       File "/usr/lib/python3.13/site-
packages/sphinx/config.py", line 354, in read
[tutorial ]         raise ConfigError(
[tutorial ]             __("config directory doesn't contain a conf.py
file (%s)") % confdir
[tutorial ]         )
[tutorial ]     sphinx.errors.ConfigError: config directory doesn't
contain a conf.py file (/build/sagemath-doc-
git/src/build/src/doc/ru/tutorial)
[tutorial ] The full traceback has been saved in:
[tutorial ] /tmp/sphinx-err-6fr5ejwk.log
[tutorial ] To report this error to the developers, please open an issue
at <https://github.com/sphinx-doc/sphinx/issues/>. Thanks!
[tutorial ] Please also report this if it was a user error, so that a
better error message can be provided next time.
```


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- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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<!-- - sagemath#12345: short description why this is a dependency -->
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URL: sagemath#40904
Reported by: Tobias Diez
Reviewer(s):

Author: Release Manager

src/doc/meson.build

@@ -10,13 +10,6 @@ if not sphinx_check.found()
   subdir_done()
 endif
 
-# TODO: Migrate this completely to meson
-doc_bootstrap = custom_target(
-  'bootstrap',
-  output: ['autogen'],
-  command: [files('bootstrap'), meson.current_build_dir()],
-  env: {'SAGE_ROOT': root},
-)
 doc_src = []
 subdir('el')
 subdir('ca')
@@ -33,6 +26,17 @@ subdir('es')
 subdir('fr')
 subdir('ja')
 
+# TODO: Migrate this completely to meson
+doc_bootstrap = custom_target(
+  'bootstrap',
+  output: ['autogen'],
+  command: [files('bootstrap'), meson.current_build_dir()],
+  env: {'SAGE_ROOT': root},
+  # doc_src is not really a dependency of the bootstrap, but we want to make sure
+  # that all the source files are present before running the actual doc build
+  # so let's collect all source-related targets here.
+  depends: doc_src,
+)
 
 references = run_command(
   py,
@@ -79,7 +83,6 @@ foreach type : ['inventory', 'html', 'pdf']
     endif
     target = custom_target(
       'doc-' + type + '-reference-' + short_ref,
-      #input: doc_src,
       output: [type + short_ref],
       command: [
         py,