Rename docs dirs and files to have more digits (clockworklabs#3851)

# Description of Changes

This makes it possible to insert new pages between existing pages
without renaming the existing pages.

I also added a section to docs/README.md with procedure for how to name
new pages, which I'll paste here:

### Adding new pages

All of our directory and file names are prefixed with a five-digit
number which determines how they're sorted.
We started with the hundreds place as the smallest significant digit, to
allow using the tens and ones places to add new pages between.
When adding a new page in between two existing pages, choose a number
which:

- Doesn't use any more significant figures than it needs to.
- Is approximately halfway between the previous and next page.

For example, if you want to add a new page between `00300-foo` and
`00400-bar`, name it `00350-baz`. To add a new page between `00350-baz`
and `00400-bar`, prefer `00370-quux` or `00380-quux`, rather than
`00375-quux`, to avoid populating the ones place.

To add a new page after all previous pages, use the smallest multiple of
100 larger than all other pages. For example, if the highest-numbered
existing page is `01350-abc`, create `01400-def`.

# API and ABI breaking changes

N/a

# Expected complexity level and risk

1 - @JulienLavocat said this wouldn't break anything.

# Testing

None.

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Co-authored-by: Jason Larabie <jason@clockworklabs.io>

Author: gefjon