gh-40587: Minor documentation formatting improvement
    
And redirect readers to the page. I think developers are most likely to
want to build the documentation.

I think we should ignore the test-new failure, which is likely an
oversight introduced in #39641 . As
long as the corresponding test pass in test-long it should be fine.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [ ] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
- [ ] 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

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
    
URL: #40587
Reported by: user202729
Reviewer(s): Tobias Diez

Author: Release Manager

pkgs/sage-docbuild/README.rst

@@ -2,6 +2,11 @@
  Sage: Open Source Mathematics Software: Build system of the Sage documentation
 ================================================================================
 
+.. NOTE::
+
+   If you are a developer and want to build the SageMath documentation from source,
+   refer to `developer's guide <../../developer/sage_manuals.html>`_.
+
 About SageMath
 --------------
 

src/doc/en/developer/sage_manuals.rst

@@ -77,7 +77,9 @@ function's documentation.  Type::
 Hyperlinks
 ==========
 
-The documentation can contain links toward modules, classes, or methods, e.g.::
+The documentation can contain links toward modules, classes, or methods, e.g.:
+
+.. CODE-BLOCK:: rest
 
     :mod:`link to a module <sage.module_name>`
     :mod:`sage.module_name` (here the link's text is the module's name)
@@ -335,7 +337,7 @@ links that it contains, use the ``--warn-links`` flag. Note that Sphinx will not
 rebuild a document that has not been updated, and thus not report its broken
 links::
 
-        sage --docbuild --warn-links reference html
+    sage --docbuild --warn-links reference html
 
 .. _section-manuals-names:
 

src/sage_docbuild/builders.py

@@ -2,6 +2,11 @@
 """
 Documentation builders
 
+.. NOTE::
+
+   If you are a developer and want to build the SageMath documentation from source,
+   refer to `developer's guide <../../../developer/sage_manuals.html>`_.
+
 This module is the starting point for building documentation, and is
 responsible to figure out what to build and with which options. The actual
 documentation build for each individual document is then done in a subprocess
@@ -21,7 +26,7 @@
 in ``local/share/inventory``.
 
 The reference manual is built in two passes, first by :class:`ReferenceBuilder`
-with ``inventory`` output type and secondly with``html`` output type. The
+with ``inventory`` output type and secondly with ``html`` output type. The
 :class:`ReferenceBuilder` itself uses :class:`ReferenceTopBuilder` and
 :class:`ReferenceSubBuilder` to build subcomponents of the reference manual.
 The :class:`ReferenceSubBuilder` examines the modules included in the
@@ -318,7 +323,7 @@ def clean(self, *args):
 
 def build_many(target, args, processes=None):
     """
-    Thin wrapper around `sage_docbuild.utils.build_many` which uses the
+    Thin wrapper around :func:`sage_docbuild.utils.build_many` which uses the
     docbuild settings ``NUM_THREADS`` and ``ABORT_ON_ERROR``.
     """
     if processes is None: