Add numberstring and more detailed indicator unit notes

[cjrace]

Overview of changes

Adding numberstring as an acceptable indicator_unit.

Why are these changes being made?

We'd added it into EES this week, so the guidance should match to tell users it's an option.

Detailed description of changes

Added a new table that lists all acceptable values in it with a quick description of what they do.

Weirdly, I think the missing TOC issue might be fixed as a result of these changes now too? @rmbielby @jen-machin

Issue ticket number/s and link

None

Checklist before requesting a review

• [x] I have checked the contributing guidelines
• [x] I have checked for and linked any relevant issues that this may resolve
• [x] I have checked that these changes build locally
• [x] I understand that if merged into main, these changes will be publicly available

[cjrace]

Update analysts guide with numberstring guidance