Add new User's Guide sub-sections: Writing assignments

- [ ] This is the Writing umbrella issue that lists Section/Chapter author-assignments as sub-issues. Do NOT break the organization of issues that already have parents, unless it makes sense to move them here from the Reviewing umbrella.
- [ ] RETITLED THE ISSUE BEFORE ASSIGNING ALL User's Guide WRITING, because assignments may continue for a while.

[List of docs-loc:users-guide.](https://github.com/orgs/ESCOMP/projects/54/views/1?filterQuery=-is%3Aclosed+-is%3Apr+-type%3AParent+-status%3A%22Not+for+6.0%22&sliceBy%5Bvalue%5D=docs-loc%3Ausers-guide)