Document dns_nsupdate provider in managing DNS section

[ekohl]

This takes the idea that there should be a central place with documentation on all DNS providers. Not spread over multiple guides. It starts with the dns_nsupdate provider, but this should be completed with all of the providers.

This is split off from https://github.com/theforeman/foreman-documentation/pull/2933 to keep the scope limited. Infoblox turned out to be much harder than expected.

• [x] I am okay with my commits getting squashed when you merge this PR.
• [x] I am familiar with the contributing guidelines.

Please cherry-pick my commits into:

• [x] Foreman 3.11/Katello 4.13
• [x] Foreman 3.10/Katello 4.12
• [ ] Foreman 3.9/Katello 4.11 (Satellite 6.15; orcharhino 6.8/6.9)
• [ ] Foreman 3.8/Katello 4.10
• [ ] Foreman 3.7/Katello 4.9 (Satellite 6.14)
• [ ] Foreman 3.6/Katello 4.8
• [ ] Foreman 3.5/Katello 4.7 (Satellite 6.13; orcharhino 6.6/6.7)
• We do not accept PRs for Foreman older than 3.5.

[github-actions[bot]]

The PR preview for acc5aeb7c62042de6bc61f9c4cc5569618b7f8af is available at theforeman-foreman-documentation-preview-pr-3200.surge.sh

The following output files are affected by this PR:

• Installing_Proxy/index-foreman-el.html
• Installing_Proxy/index-katello.html
• Installing_Proxy/index-orcharhino.html
• Installing_Proxy/index-satellite.html

show diff

show diff as HTML

[ekohl]

I should have noted that we already sort of have this documented in https://docs.theforeman.org/nightly/Installing_Server/index-katello.html#reverting-to-internal-dns-service_foreman (where I just noticed we have a dead anchor) and https://docs.theforeman.org/nightly/Installing_Server/index-katello.html#configuring-dns-dhcp-and-tftp_foreman. My primary aim was to provide readers with an easy way to set up only DNS instead of configuring DHCP & TFTP as well since they don't need to run on the same server. My general recommendation is to aim for a single DNS Smart Proxy unless you have a specific need for multiple.

[ekohl]

Rebased to resolve conflicts and I included @asteflova's suggestions, except the header one.

[ekohl]

cd7b2a1ff3..691fc5736f 3.12 -> 3.12

And 3.11 had a conflict so I'll keep this to 3.12+.