chore: move compass out of docs/, which pkgdown deletes

compass.toml, JOURNAL.md, STATUS.md and decisions/ were sitting inside pkgdown's
output directory. Asked directly, pkgdown listed docs/pm and docs/decisions among
the 28 top-level entries clean_site() would delete, and the guard that would have
refused -- check_dest_is_pkgdown() -- was satisfied by docs/pkgdown.yml. After the
move it lists 26 and none of them are compass's.

Nothing was lost. The journal had no entries and there were no decision records
yet, so this was the cheapest moment to move.

The .gitignore workaround goes with it. Re-including two children of an excluded
docs/ forced the rule to be written as /docs/* plus two negations, which changed
the anchoring and made the fixture corpus's own docs/ need a separate rule. A bare
docs/ matches at any depth again, so both are unnecessary.

.Rbuildignore gains ^pm$ -- R CMD check flags a non-standard top-level directory.

Compass reads both layouts, so this repository worked either way; the point is
that docs/ is a directory another tool empties.

.gitignore

@@ -4,17 +4,11 @@
 .Ruserdata
 *.Rproj
 inst/doc
-# pkgdown output. Listed as children rather than `docs/` so compass's
-# docs/pm/ and docs/decisions/ can be re-included -- git cannot re-include
-# anything beneath an excluded directory.
-#
-# Note the anchoring change this forces: a bare `docs/` matches a directory of
-# that name at ANY depth, while `/docs/*` matches only at the repo root. The
-# fixture corpus's own docs/ therefore needs its own rule to stay excluded.
-/docs/*
-!/docs/pm/
-!/docs/decisions/
-inst/extdata/fixture_corpus/docs/
+# pkgdown output. Compass used to keep its files in docs/pm/ and
+# docs/decisions/, which forced this to be written as children with two
+# re-includes -- git cannot re-include anything beneath an excluded directory.
+# Compass lives in pm/ now, so the whole directory can be excluded again.
+docs/
 /doc/
 /Meta/
 .DS_Store