Add redirects for all pages moved in #1439

[kenjenkins]

The updates to _redirects in #1439 appear to be incomplete. Every moved page should have a corresponding redirect.

[kenjenkins]

I think this incantation will show all the pages that were deleted or moved:

% git show 1d26b1be81bf909e857f86acfc3be7f02ce6dce4 --diff-filter=D --no-renames --name-only | sed -nr 's/^content(.*)\.mdx?$/\1/p' 
/docs/concepts/certificates
/docs/deploy/clients
/docs/deploy/clients/pomerium-cli
/docs/deploy/clients/pomerium-desktop
/docs/deploy/core
/docs/deploy/core/binary
/docs/deploy/core/changelog
/docs/deploy/core/from-source
/docs/deploy/core/upgrading
/docs/deploy/enterprise
/docs/deploy/enterprise/changelog
/docs/deploy/enterprise/configure
/docs/deploy/enterprise/install
/docs/deploy/enterprise/quickstart
/docs/deploy/enterprise/upgrading
/docs/deploy/k8s/configure
/docs/deploy/k8s/ingress
/docs/deploy/k8s/install
/docs/deploy/k8s/quickstart
/docs/deploy/k8s/reference
/docs/deploy/production-deployment
/docs/deploy/releases
/docs/deploying
/docs/guides/js-sdk
/docs/guides/local-oidc
/docs/guides/securing-tcp
/docs/zero/cluster-status
/docs/zero/upgrading

We can loop over the output to check for any paths that return an HTTP error code:

% git show 1d26b1be81bf909e857f86acfc3be7f02ce6dce4 --diff-filter=D --no-renames --name-only | sed -nr 's/^content(.*)\.mdx?$/\1/p' | while read p; do (curl -sf https://www.pomerium.com$p > /dev/null || echo "Missing path $p"); done
Missing path /docs/deploy/clients
Missing path /docs/deploy/production-deployment
Missing path /docs/zero/upgrading

[ZPain8464]

From DocsSync:

• docs/deploy/clients: The missing client page should point to the clients.md file.
• /docs/zero/upgrading: This page was only available in main, not in 0-26-0.
• /docs/deploy/production-deployment: There was some discussion on converting this page into a more comprehensive and prescriptive document for sales and marketing.

TODOs:

• I'll bring up the Upgrading and Production Deployment pages in our internal Slack to get the team's input.
• I'll add a redirect to point the old clients path to the new one.

[ZPain8464]

#1482 adds a redirect for the Zero Upgrading page. The last pending page would be the Production Deployment page.