Misc: Remove pykdtree dependency, codecov config, SPEC 0 tooling and other changes

[VeckoTheGecko]

Several small changes:

• doc: add release link to navbar
• doc: update warning message on documentation page (related to tutorial_diffusion: SmagDiff example not working #1665)
• doc: clarify timedelta conversion in tutorial_output notebook
• Remove pykdtree dependency (fixes Two libraries used for KDTree in Parcels. Use only one? #1616)
• Update codecov config to enable PR comments
• Add reference file (along with check of that file) for compliance with SPEC 0 (completes Drop Python 3.8, 3.9 support and expand CI unit testing #1658)

[VeckoTheGecko]

This is the output from the min-deps check workflow

Package           Required             Policy               Status
----------------- -------------------- -------------------- ------
python            3.10    (2021-10-04) 3.9     (2020-10-05) > (!)
cftime            1.6     (2022-03-03) 1.6     (2022-03-03) =
cgen              2020.1  (2020-02-07) 2020.1  (2020-02-07) =
dask              2022.8  (2022-08-05) 2022.8  (2022-08-05) =
matplotlib-base   3.5     (2021-11-18) 3.5     (2021-11-18) =
netcdf4           1.6     (2022-06-29) 1.6     (2022-06-29) =
numpy             1.23    (2022-06-23) 1.23    (2022-06-23) =
platformdirs      2.5     (2022-02-10) 2.5     (2022-02-10) =
psutil            5.9     (2021-12-30) 5.9     (2021-12-30) =
pymbolic          2022.1  (2022-05-09) 2022.1  (2022-05-09) =
pytest            7.1     (2022-03-14) 7.1     (2022-03-14) =
scipy             1.9     (2022-07-30) 1.9     (2022-07-30) =
trajan            0.1     (2022-11-22) 0.1     (2022-11-22) =
tqdm              4.64    (2022-04-04) 4.64    (2022-04-04) =
xarray            2022.6  (2022-07-23) 2022.6  (2022-07-23) =
zarr              2.12    (2022-06-23) 2.12    (2022-06-23) =

Currently the workflow gets run every 3 months. If a dependency in the reference file is out of date according to SPEC 0 it gets flagged in that table and CI gets failed.

(note Python gets flagged since we're technically slightly misaligned with SPEC 0 by pinning to 3.10. Come October we'll be in sync (and I wanted to use Python 3.10 features))