With the goal of making `toPretty` suitable for rendering option
values, render derivations as `<derivation foo-1.0>` instead of
`<derivation /nix/store/…-foo-1.0.drv>`.

This is to avoid causing sudden evaluation errors for out-of-tree
projects that have options with `default = pkgs.someUnfreePackage;` and
no `defaultText`.

Render un`_type`d defaults and examples as `literalExpression`s using
`lib.generators.toPretty` so that consumers don't have to reinvent Nix
pretty-printing. `renderOptionValue` is kept internal for now intentionally.

Make `toPretty` print floats as valid Nix values (without a tilde).

Get rid of the now-obsolete `substSpecial` function.

Move towards disallowing evaluation of packages in the manual by
raising a warning on `pkgs.foo.{outPath,drvPath}`; later, this should
throw an error. Instead, module authors should use `literalExpression`
and `mkPackageOption`.

This variable is set to "master" on master and "release-YY.DD" on the
release branches so that the manual can link to the correct revision.

A generic function to convert a path into a documentation-friendly
{path, source, url} triple based on a list of file sources.

Provides a mechanism for rendering paths relative to a known prefix
using angle bracket syntax. Backed by `lib.strings.lookupPrefix`.

Add an internal `_module.sources` option which specifies a list of
sources to be used by `lib.strings.lookupPrefix` to render paths in
option values and declarations.
The sources always include the `nixpkgsSource`, which links to the
manual for `lib.trivial.manualRevision`.

Implement this logic in a new `lib.optionsToDocTemplate` function which
deprecates `lib.optionAttrSetToDocList`. The returned option
declarations are now attrsets as returned by `lookupPrefix`.
If `decl.source` is set, then the declaration is rendered using angle
bracket syntax: `<${decl.source}/${decl.path}>`. Other consumers are
free to treat the source name as they intend.

Deprecate `documentation.nixos.extraModuleSources`, converting it to
`_module.sources` with `name` set to `_unknown`.

We now call `eval-cacheable-options.nix` with a single filtered nixpkgs
source, which is easier to maintain than a bunch of paths and allows
things to work more smoothly (mainly `lib.trivial.release` can access
../.version, and `lib.nixpkgsSource` works as intended so we don't
need to special-case anything).