[DOCS] Add 8.0.0-rc1 release notes

[jrodewig]

No description provided.

[jrodewig]

Going to merge this in, but a post-merge 👍 is appreciated. Thanks!

[masseyke]

I think the release notes are missing references to the following:
Bugs:

• Correctly reading empty fields in as null rather than throwing exception #1816
• resolve saveToEs saves case classes fields with NULL values #998 #1478
• Improving documentation about es.read.fields.include, and fixing a related bug #1822

Features:

• Adding support for the date_nanos field type #1803

[jrodewig]

I think the release notes are missing references to the following: Bugs:

• Correctly reading empty fields in as null rather than throwing exception #1816
• resolve saveToEs saves case classes fields with NULL values #998 #1478
• Improving documentation about es.read.fields.include, and fixing a related bug #1822

Features:

• Adding support for the date_nanos field type #1803

Thanks @masseyke. I excluded these because they had the v7.17 label, but 7.17 hasn't actually been released yet... so these should be in the 8.0.0-rc1 RNs. I've opened #1859 to add these missing items.

That PR also adds #1828, which isn't in this list.