docs(conf.py): name the rolling docs as "rolling (current)", not stale circinus

The `current` branch's `release` literal was `'1.5.x (circinus)'` —
stale since circinus split off as its own LTS branch. The rendered
llms.txt verification on the RTD preview surfaced the bug as the
canonical version label of the rolling docs:

  > This documentation covers 1.5.x (circinus).

…on a build whose URLs are `/en/rolling/`. The new build-time render
introduced by this PR makes the inconsistency a user-visible artifact
exposed to LLM consumers, so fix the source-of-truth literal here.

Now `release = 'rolling (current)'` and `version = 'rolling'`. Sphinx
also exposes `release` in the page footer ("v: rolling (current)")
and search metadata, so fixing this string is a single source-of-truth
change for the rolling docs.

Sagitta and circinus `release` strings are correct already
(`'1.4.x (sagitta)'` and `'1.5.x (circinus)'` respectively); no
cross-branch sweep needed for `release`.

Addresses CodeRabbit review feedback on PR #1874 (the originally pushed-back
release-mismatch finding — verification proved the push-back wrong).

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