Use "release candidate" in book (#2920)

## Issue Addressed

NA

## Proposed Changes

Since we use the `rc` (release candidate) tag in our version strings, it seems consistent if we also use "release candidate" in the book rather than "pre-release".

Notably, Github adds a "pre-release" tag to release when we request. I think it's OK that Github uses that term whilst we consistently use "release candidate". Our docs indicate that the terms are interchangeable. 

## Additional Info

I hope to use the new docs link in the `v2.1.0` release, so it would be nice if we can merge this soon 🙏

Author: paulhauner

book/src/advanced-pre-releases.md

@@ -1,40 +1,4 @@
 # Pre-Releases
 
-[sigp/lighthouse]: https://github.com/sigp/lighthouse
-[sigp/lighthouse/releases/latest]: https://github.com/sigp/lighthouse/releases/latest
-[sigp/lighthouse/releases]: https://github.com/sigp/lighthouse/releases
-[`v1.4.0-rc.0`]: https://github.com/sigp/lighthouse/releases/tag/v1.4.0-rc.0
-[`v1.4.0`]: https://github.com/sigp/lighthouse/releases/tag/v1.4.0
-
-From time-to-time, Lighthouse *pre-releases* will be published on the [sigp/lighthouse] repository.
-These releases have passed the usual automated testing, however the developers would like to see it
-running "in the wild" in a variety of configurations before declaring it an official, stable
-release. Pre-releases are also used by developers to get feedback from users regarding the
-ergonomics of new features or changes.
-
-Github will clearly show such releases as a "Pre-release" and they *will not* show up on
-[sigp/lighthouse/releases/latest]. However, pre-releases *will* show up on the
-[sigp/lighthouse/releases] page, so **please pay attention to avoid the pre-releases when you're
-looking for stable Lighthouse**.
-
-### Examples
-
-[`v1.4.0-rc.0`] has `rc` (release candidate) in the version string and is therefore a pre-release. This
-release is *not* stable and is *not* intended for critical tasks on mainnet (e.g., staking).
-
-However, [`v1.4.0`] is considered stable since it is not marked as a pre-release and does not
-contain `rc` in the version string. This release is intended for use on mainnet.
-
-## When to use a pre-release
-
-Users may wish to try a pre-release for the following reasons:
-
-- To preview new features before they are officially released.
-- To help detect bugs and regressions before they reach production.
-- To provide feedback on annoyances before they make it into a release and become harder to change or revert.
-
-## When *not* to use a pre-release
-
-It is not recommended to use pre-releases for any critical tasks on mainnet (e.g., staking). To test
-critical features, try one of the testnets (e.g., Prater).
-
+Pre-releases are now referred to as [Release Candidates][./advanced-pre-releases.md]. The terms may
+be used interchangeably.

book/src/advanced-release-candidates.md

@@ -0,0 +1,43 @@
+# Release Candidates
+
+[sigp/lighthouse]: https://github.com/sigp/lighthouse
+[sigp/lighthouse/releases/latest]: https://github.com/sigp/lighthouse/releases/latest
+[sigp/lighthouse/releases]: https://github.com/sigp/lighthouse/releases
+[`v1.4.0-rc.0`]: https://github.com/sigp/lighthouse/releases/tag/v1.4.0-rc.0
+[`v1.4.0`]: https://github.com/sigp/lighthouse/releases/tag/v1.4.0
+
+From time-to-time, Lighthouse *release candidates* will be published on the [sigp/lighthouse]
+repository. These releases have passed the usual automated testing, however the developers would
+like to see it running "in the wild" in a variety of configurations before declaring it an official,
+stable release. Release candidates are also used by developers to get feedback from users regarding the
+ergonomics of new features or changes.
+
+Github will clearly show such releases as a "Pre-release" and they *will not* show up on
+[sigp/lighthouse/releases/latest]. However, release candidates *will* show up on the
+[sigp/lighthouse/releases] page, so **please pay attention to avoid the release candidates when
+you're looking for stable Lighthouse**.
+
+From time to time, Lighthouse may use the terms "release candidate" and "pre release"
+interchangeably. A pre release is identical to a release candidate.
+
+### Examples
+
+[`v1.4.0-rc.0`] has `rc` in the version string and is therefore a release candidate. This release is
+*not* stable and is *not* intended for critical tasks on mainnet (e.g., staking).
+
+However, [`v1.4.0`] is considered stable since it is not marked as a release candidate and does not
+contain `rc` in the version string. This release is intended for use on mainnet.
+
+## When to use a release candidate
+
+Users may wish to try a release candidate for the following reasons:
+
+- To preview new features before they are officially released.
+- To help detect bugs and regressions before they reach production.
+- To provide feedback on annoyances before they make it into a release and become harder to change or revert.
+
+## When *not* to use a release candidate
+
+It is not recommended to use release candidates for any critical tasks on mainnet (e.g., staking).
+To test critical features, try one of the testnets (e.g., Prater).
+