prepare for the 6.2.0 release

[kevinushey]

Release preparation for RcppParallel 6.2.0. The functional work landed
earlier in #267-#275; this is the packaging, NEWS, and documentation pass
on top of it.

Changes

• .Rbuildignore: exclude .worktrees. R CMD build was sweeping local
  git worktrees into the source tarball -- 1741 of its 2193 files, 3.9 MB vs
  710 KB. This would have failed the CRAN submission.
• NEWS.md: consolidate the cycle into a 6.2.0 section. Three entries
  described intermediate states that later commits superseded, and
  contradicted the entries above them: one said configure prefers an Rtools
  TBB when it is oneTBB (it no longer probes for one at all), and two
  described the tbb.dll stub as present (it was removed in build the bundled onetbb as a shared library on windows #274).
• DESCRIPTION: 6.1.1.9000 -> 6.2.0.
• R/flags.R, man/flags.Rd: the details section claimed LdFlags()
  emits nothing on macOS. That stopped being true in 5.1.8, and this release
  adds an -rpath there, so the text now describes each platform.
• R/tbb.R: correct the comment on the TBB_ENABLED guard -- it referred
  to a tinythread build outcome that require cmake on all platforms; tbb is always enabled #275 removed. The guard itself stays.
• patches/windows_arm64.diff: delete. It patches src/tbb/build/Makefile.*,
  removed with the make-based TBB in 6.0.0, and its one live hunk is now
  upstream in oneTBB 2022. It is the only patch that fails a reverse-apply
  check against the vendored tree.
• .github/workflows/R-CMD-check.yaml: fix two comments claiming R 4.2 is
  the only Windows config that builds the bundled oneTBB. Since require cmake on all platforms; tbb is always enabled #275, every
  Windows config does.
• cran-comments.md: new, for the submission.

Verification

• R CMD check --as-cran on RcppParallel_6.2.0.tar.gz: 1 NOTE, the
  incoming-feasibility days-since-last-update note. (An earlier
  non-portable-flags WARNING traced to a local ~/.R/Makevars, not the
  package.)
• CI green across all 9 jobs, including the three Windows downstream
  link checks.
• cmake resolves from within Rtools on all three Windows configs
  (rtools42, rtools45, rtools45-aarch64), confirming the "cmake required
  everywhere" change in require cmake on all platforms; tbb is always enabled #275 does not strand Windows users.
• rstan built from source and fitted a model on Rtools42 / R 4.2 Windows
  (run 30473955617) --
  the configuration that motivated the Windows rework. The version guard
  confirms it tested this build rather than CRAN's.

Outstanding

• cran-comments.md has a TODO for the revdepcheck section; the cloud job
  against 6.2.0 is still running.