doc: DOCSP-54251 & DOCSP-54252 -- Document how to move from or to Service Accounts authentication

[xargom]

Description

DOCSP-54251

DOCSP-54252

Creates a guide to move to Service Accounts auth and updates the landing page with SA as the first auth option.

Link to any related issue(s):

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• This change requires a documentation update
• [ x ] Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contributing guides
• I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• If changes include deprecations or removals I have added appropriate changelog entries.
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

[github-actions]

APIx bot: a message has been sent to Docs Slack channel

[marcosuma]

Q: what do we mean with this?

[xargom]

Definitely the phrasing is not the better (changes incoming), but there are 3 ideas here:

1. The token expires.
2. Redirect the user to the Atlas SA docs.
3. "the MongoDB Terraform provider does not support additional Token OAuth features." I'm explicitly stating a suggestion mentioned in the project scope doc: https://docs.google.com/document/d/1PuAwTTNbVLUsqMH9wmmHKUCbEojhkQ_oHL2ONxwXZhs/edit?tab=t.0#heading=h.5c9t51nomy3f. However, i'm not sure if we actually need to have this in the final version.

[marcosuma]

we need @bodegus inputs here, can you reach out to him?

[xargom]

Added the changes related to DOCSP-54252 since it seemed easier to manage it all in one PR.

[lantoli]

should we also mention the 10 token / minute rate limiting when using SA client_id and client_secret?

[xargom]

I don't see why not. I added it.

[xargom]

@lantoli Do the auth methods currently described in the landing page use PAK? If so, and the idea is to discourage using PAK, should we replace them all with SA information?

Just trying to figure out if mixing PAK and SA info in the index may lead to confusions.