README: describe the calendar as it is built today

`build.sh` no longer post-processes anything: since the switch to ICS files it
just deletes `public/` and runs `hugo`. The `CALENDAR` placeholder, the Python
dependency on `icalendar`, `python-dateutil` and `pytz`, and the `de_DE.UTF-8`
locale requirement are all gone, and nothing in `packages.nix`, `devShells.nix`
or `flake.nix` pulls in Python any more. Drop those claims.

Describe instead where the two calendar views actually come from: the
"Nächste Veranstaltungen" table and the calendar page are rendered in the
browser from `/calendars/all.ics`, which is published next to the site and is
not built from this repository. That also replaces the old advice to preview
them via `./build.sh` plus a local HTTP server, which no longer helps.

Without that file the two of them stay empty. Hugo serves everything below
`static/` from the root of the site, so it can simply be put there, and the
command that downloads it is worth writing down. Checked from an empty state:
afterwards the file is served under `/calendars/all.ics` by `hugo serve` and the
table on the start page fills with the upcoming events. It is a copy of
published data and has no business in the repository, so ignore it.

Mention the per-section `.ics` feeds Hugo does generate, so the ICS templates
under `layouts/` are not mistaken for the source of `all.ics`.

Fixes: c28f04c6e8 ("switch to ics files; make calendars work; fix some minor issues")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

.gitignore

@@ -202,3 +202,8 @@ $RECYCLE.BIN/
 *.lnk
 
 # End of https://www.toptal.com/developers/gitignore/api/windows,linux,macos,hugo,pycharm+all,vim,direnv
+
+### CCCB ###
+# The calendar the site reads its events from is published next to the site and
+# only downloaded here to look at the site locally, see the README.
+/static/calendars/