docs: add docs for admin endpoint auth

[abhimanyusinghgaur]

Fixes #GRAPHQL-523.

This PR adds documentation about the required authentications to access the admin endpoints.
Please see this discuss post for more details: https://discuss.dgraph.io/t/authentication-for-admin-endpoints/6786

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[vardhanapoorv]

Fix the formatting https://dgraph-5a7ab2e30b-75476.surge.sh/deploy/#dgraph-administration, otherwise minor comments.

Reviewable status: 0 of 2 files reviewed, 3 unresolved discussions (waiting on @abhimanyusinghgaur, @danielmai, and @MichaelJCompton)

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wiki/content/deploy/index.md, line 2421 at r1 (raw file):

  1. IP White-listing, if `--whitelist` flag is passed to alpha.
  2. Poor-man's auth, if `--auth_token` flag is passed to alpha (means you will need to pass the `auth_token` as `X-Dgraph-AuthToken` header while making the HTTP request if this is enabled).
  3. Guardian only access, if ACL is enabled (means you need to pass the ACL JWT of a Guardian user as `X-Dgraph-AccessToken` header while making the HTTP request if this is enabled).

the ACL, JWT
can drop "if this is enabled" since already specified

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wiki/content/deploy/index.md, line 2423 at r1 (raw file):

  3. Guardian only access, if ACL is enabled (means you need to pass the ACL JWT of a Guardian user as `X-Dgraph-AccessToken` header while making the HTTP request if this is enabled).

Admin endpoint means any http endpoint which provides admin functionalities. Normally, the path starts with `/admin` for such endpoints, except a few. So, at present this list includes:

So, currently ...

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wiki/content/deploy/index.md, line 2435 at r1 (raw file):

There are a few exceptions to the general rule described above:
1. `/login`: This endpoint logs-in an ACL user, and provides them with JWT. Only IP Whitelisting and Poor-man's auth checks are performed for this. As one won't be able to login using ACL if we mandate Guardian only access on this.

a JWT

[pawanrawal]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2.
Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @abhimanyusinghgaur, @danielmai, and @MichaelJCompton)

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wiki/content/deploy/index.md, line 2421 at r2 (raw file):

1. IP White-listing, if `--whitelist` flag is passed to alpha.
2. Poor-man's auth, if `--auth_token` flag is passed to alpha (means you will need to pass the `auth_token` as `X-Dgraph-AuthToken` header while making the HTTP request).

if alpha is started with the --auth_token flag

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wiki/content/deploy/index.md, line 2438 at r2 (raw file):

There are a few exceptions to the general rule described above:

1. `/login`: This endpoint logs-in an ACL user, and provides them with a JWT. Only IP Whitelisting and Poor-man's auth checks are performed for this. As one won't be able to login using ACL if we mandate Guardian only access on this.

You can remove the last line, As one won't be able to...