docs: add a language pack "contract" document

[lance]

Rendered version: https://github.com/lance/func/blob/lance/add-language-pack-contract/docs/language-pack-providers/language-pack-contract.md

The docs/guides/langugage-packs.md document does a good job of describing what's required of a language pack in terms of directory structure and the options for the manifest.yaml file. But it does a pretty crappy job of describing the whole thing in context. I've tried to do that here.

Also some fixes to links and reorganization of the docs to make things a little easier to find.

Fixes: #574 (maybe?)

Signed-off-by: Lance Ball lball@redhat.com

[knative-prow-robot]

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This pull-request has been approved by: lance

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[codecov]

Codecov Report

Merging #918 (4b4129a) into main (739cded) will increase coverage by 1.98%.
The diff coverage is 29.13%.

@@            Coverage Diff             @@
##             main     #918      +/-   ##
==========================================
+ Coverage   43.85%   45.83%   +1.98%     
==========================================
  Files          55       57       +2     
  Lines        5163     7442    +2279     
==========================================
+ Hits         2264     3411    +1147     
- Misses       2576     3703    +1127     
- Partials      323      328       +5     

Impacted Files	Coverage Δ
cmd/completion_util.go	0.00% <0.00%> (ø)
cmd/invoke.go	34.81% <0.00%> (-0.97%)	⬇️
docker/docker_client.go	82.52% <ø> (+0.57%)	⬆️
k8s/logs.go	0.00% <0.00%> (ø)
k8s/secrets.go	13.54% <0.00%> (-0.75%)	⬇️
s2i/builder.go	32.69% <32.69%> (ø)
client.go	61.94% <50.00%> (+2.01%)	⬆️
invoke.go	52.99% <61.53%> (+2.99%)	⬆️
cmd/build.go	60.49% <70.83%> (+0.86%)	⬆️
cmd/root.go	72.28% <100.00%> (+1.66%)	⬆️
... and 55 more

---

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[lance]

Converting to draft, as it's still rough and unfinished in a couple of spots - still looking for feedback.
Ready for review

[lkingland]

Hugely beneficial addition! All I could come up with was a bunch of minor syntactic nits.

👍🏻

/hold for others

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lance, lkingland

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[lance]

@lkingland thanks for the thorough review. I've incorporated your feedback, with a couple of replies to your comments above. I've also inlined the language pack reference as suggested. It is mostly a copy/paste, but I did tweak some of the wording here and there to try and make things clearer. PTAL

[lkingland]

/lgtm

[lance]

/unhold