Fix dead links in docs and add linter

[adriangonz]

What this PR does / why we need it:
Fix most warnings thrown by Sphinx, some dead links, and upgrade the versions of the docs' dependencies. This PR also adds a /docs command which runs some linters on the docs and also crawls all external links to verify they are working.

Which issue(s) this PR fixes:

Fixes #1760

Special notes for your reviewer:

• The external link crawling is a bit heavy, so we probably can't run it on every PR. That's why I've split into a separate /docs command.
• I've replaced all links between notebooks outside of the /doc folder for absolute links to the Seldon Core docs. This is to work around a limitation on nbsphinx-link.
• I've removed all notebooks' static assets from the /doc folder to use instead the extra-media key on the nblink file.

Does this PR introduce a user-facing change?:

NONE

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[seldondev]

Thu Oct 8 16:47:08 UTC 2020
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[seldondev]

Thu Oct 8 16:47:23 UTC 2020
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[seldondev]

Thu Oct 8 16:59:13 UTC 2020
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[seldondev]

Thu Oct 8 16:59:17 UTC 2020
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[adriangonz]

/test docs

[seldondev]

Thu Oct 8 17:06:08 UTC 2020
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[seldondev]

Thu Oct 8 17:06:08 UTC 2020
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[seldondev]

Thu Oct 8 17:09:13 UTC 2020
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[adriangonz]

/cc @cliveseldon @axsaucedo @RafalSkolasinski

[axsaucedo]

This is great! Looks good! Seems there may need to be a small change on the image as build looks like pandoc is missing, but other than that looks great!

[adriangonz]

@axsaucedo good spot! I've now pushed a new version of the image.

[seldondev]

Fri Oct 9 11:17:10 UTC 2020
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[seldondev]

Fri Oct 9 11:17:22 UTC 2020
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[adriangonz]

/test docs

[seldondev]

Fri Oct 9 11:20:16 UTC 2020
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[adriangonz]

/test docs

[seldondev]

Fri Oct 9 13:02:35 UTC 2020
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Fri Oct 9 13:02:50 UTC 2020
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Fri Oct 9 13:03:52 UTC 2020
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[axsaucedo]

/approve

[axsaucedo]

@adriangonz seems there are some merge conflicts, but other than that should be good to go

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: axsaucedo

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[seldondev]

Fri Oct 9 16:27:38 UTC 2020
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Fri Oct 9 16:27:53 UTC 2020
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