Rule permalinks

[jakearchibald]

Depends on #48.

#48 and similar changes break rule references. After #48, docs that refer to "rule 22 of the GraphQL design tutorial" will be referring to the wrong rule. This change aims to future-proof the rules by providing permalinks, making reference more stable.

GitHub requires that IDs start with user-content for whatever reason.

[gmac]

[swalkinshaw]

I'm not sure about this as it makes the plain markdown much less readable

[jakearchibald]

@swalkinshaw I wonder if a bit of formatting could improve that.

So, instead of:

*<a href="#user-content-use-weaker-types-in-complex-cases"
id="user-content-use-weaker-types-in-complex-cases">Rule:</a> Use weaker types
for inputs (e.g. `String` instead of `Email`) when the format is unambiguous and
client-side validation is complex. This lets the server run all non-trivial
validations at once and return the errors in a single place in a single format,
simplifying the client.*

It could be:

*<a href="#user-content-use-weaker-types-in-complex-cases" id="user-content-use-weaker-types-in-complex-cases">Rule:</a>
Use weaker types for inputs (e.g. `String` instead of `Email`) when the format
is unambiguous and client-side validation is complex. This lets the server run
all non-trivial validations at once and return the errors in a single place in a
single format, simplifying the client.*

[jakearchibald]

Alternatively, the rule numbers can just be dropped, without adding the permalinks.