doc: assign ids to many headings

without stable ids on headings we cannot generate stable links to these
headings. nrd complains about this, but the current docbook workflow
does not.

a few generated ids remain, mostly in examples and footnotes. most of
the examples are generated by nixdoc (which has since gained MD export
functions, and the MD export does generate IDs).

doc/languages-frameworks/python.section.md

@@ -1117,7 +1117,7 @@ with import <nixpkgs> {};
 in python.withPackages(ps: [ps.blaze])).env
 ```
 
-#### Optional extra dependencies
+#### Optional extra dependencies {#python-optional-dependencies}
 
 Some packages define optional dependencies for additional features. With
 `setuptools` this is called `extras_require` and `flit` calls it
@@ -1801,14 +1801,14 @@ The following rules are desired to be respected:
 * Attribute names in `python-packages.nix` should be sorted alphanumerically to
   avoid merge conflicts and ease locating attributes.
 
-## Package set maintenance
+## Package set maintenance {#python-package-set-maintenance}
 
 The whole Python package set has a lot of packages that do not see regular
 updates, because they either are a very fragile component in the Python
 ecosystem, like for example the `hypothesis` package, or packages that have
 no maintainer, so maintenance falls back to the package set maintainers.
 
-### Updating packages in bulk
+### Updating packages in bulk {#python-package-bulk-updates}
 
 There is a tool to update alot of python libraries in bulk, it exists at
 `maintainers/scripts/update-python-libraries` with this repository.
@@ -1836,7 +1836,7 @@ would be:
 $ maintainers/scripts/update-python-libraries --target minor --commit --use-pkgs-prefix pkgs/development/python-modules/**/default.nix
 ```
 
-## CPython Update Schedule
+## CPython Update Schedule {#python-cpython-update-schedule}
 
 With [PEP 602](https://www.python.org/dev/peps/pep-0602/), CPython now
 follows a yearly release cadence. In nixpkgs, all supported interpreters