Rearrange documentation and add additional content

[munnerz]

What this PR does / why we need it:

This PR does a few things:

• rearranges our documentation to collate information and have a more formal structure for where info should go
• add additional content (e.g. webhook info, troubleshooting guide)
• add placeholders for more additional content still to come
• adds additional cross-links in helpful places (e.g. linking to issuer setup docs)
• collates the issuer setup guides into a single tasks/ section
• improves the installation guide with more helpful information & links out
• sets up redirects for the pages that have moved

Which issue this PR fixes fixes #1151, fixes #1269

Special notes for your reviewer:

I've setup readthedocs on my fork of cert-manager, and you can view a sample of these new docs here: https://cert-manager-munnerz.readthedocs.io/en/revamp-docs/index.html

Release note:

Overhaul documentation and add additional content

/cc @heckj @DanielMorsing

[rimusz]

cc @munnerz

[rimusz]

cc @munnerz

[rimusz]

LGTM

[jetstack-bot]

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[wwwil]

Not in the PR but theres a typo in this section: https://cert-manager-munnerz.readthedocs.io/en/revamp-docs/tasks/issuers/setup-vault.html#vault-authentication-with-a-approle

The sentence "... parameter as no effect ..." should be "... parameter has no effect ...".

Also here: https://cert-manager-munnerz.readthedocs.io/en/revamp-docs/tasks/issuers/setup-vault.html#vault-authentication-with-a-token

"... many authentication backend supported by Vault." should be "backends".
"You need to be aware that cert-manager do not refresh these tokens" should use "does not".

This sections has some issues "For testing purpose a root token which do not expire is generated at Vault installation time. WARNING: a root token should only be used for testing purpose only." I'd advise something like: "For testing purposes a root token is generated at Vault installation time. WARNING: Root tokens do not expire, so should only be used for testing purposes."

[wwwil]

It might be worth adding a note to the upgrading page about backing up first with a link to the relevant page.

[munnerz]

After discussion on the community call yesterday, we're going to cherry pick these docs changes into the v0.6 branch as they contain a fair bit of extra useful information 😄

[munnerz]

/milestone v0.6

[munnerz]

Going to merge this now as it is a net-improvement to the docs in its current state.

I'll follow up with additional PRs if anyone has any other comments, and to fill in/rewrite some areas that need it 😄