custom fields: link to custom fields docs and re-word description

[tmorrell]

Description

The heading talks about custom fields, but these aren't really custom fields since they are system defined. I added in a link to the actual custom field documentation, and adjusted the wording to be a bit clearer. The specific wording could be debated.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've targeted the master branch.
• If this documentation change impacts the current release of InvenioRDM, I will backport it to the production branch following approval or indicate to a maintainer that it should be backported.

Reminder

By using GitHub, you have already agreed to the GitHub’s Terms of Service including that:

1. You license your contribution under the same terms as the current repository’s license.
2. You agree that you have the right to license your contribution under the current repository’s license.

[fenekku]

Agreed that this section's header is incorrect and bundles up two related concepts without a thoughtful progressive explanation of them.

I'd go for either:

a) one section that covers Optional fields and custom fields in that order

## Custom fields to ## Optional fields and custom fields

With body:

InvenioRDM provides a set of ready-to-use add-on optional (custom) fields that can be enabled without advanced configuration. Those fields are grouped under: Journal, Imprint, Thesis, Meeting, and Codemeta bundles. See the [related documentation](../operate/customize/metadata/optional_fields.md) on how to enable them.

If those groups don't cover the fields you need, it's possible to extend the internal data model to fit your specific needs by creating your own custom fields in your instance. See the [related documentation](../operate/customize/metadata/custom_fields/records.md) on how to do so.

... rest of group references (Journal, ...)

OR

b) split that into 2 sections

## Optional fields

first paragraph of a)

... rest of group references (Journal, ...)

## Custom fields

second paragraph of a)

---

We want to guide the reader down to using custom fields by passing through simpler/likely solutions first so they don't go to the deep end before they need to.

I think I provided the correct link in text above also, so can be used to fix the failing CI.