docs: move and rename lifecycle override pages

[medubelko]

For #5877:

There are two how-tos that are very similar in scope, but only one is written as a task-based document.

• Move How to > Override the default build to Explanation > Build overrides.
• Rename Customize lifecycle steps and project variables to Override the build lifecycle.

---

• [x] I've followed the contribution guidelines.
• [x] I've signed the CLA.
• [x] I've successfully run make lint && make test.
• [x] I've added or updated any relevant documentation. ~~- [ ] I've updated the relevant release notes.~~

[medubelko]

@YanisaHS This should resolve #5877. Let me know what you think. :)

[YanisaHS]

@medubelko LG[ORGEOUS]TM :sparkles: