docs: add serializers documentation

[bilashcse]

What

Adds docs/serializers.md, a dedicated page documenting the serializer interface, and links it from the docsify sidebar and the README documentation list.

Why

Closes #478. As noted there, the docs described the constraints on a serializer (synchronous, returns a JSONifiable value) but never the interface itself. @davidmarkclements suggested a serializers.md page in that thread; this is that page.

What it covers

The function signature and the value a serializer receives; that serializers apply to own top-level keys of the merging object and are skipped when the value is undefined; the errorKey / err fallback; serializing the message via messageKey; child logger inheritance and override; a pointer to pino.stdSerializers; and that Symbol.for('pino.*') is deprecated in favour of formatters.log.

Behaviour described is taken from lib/tools.js (L166-173, L203-204), lib/proto.js (L115-136) and pino.js (L111-113).

Documentation only, no code changes.

[jsumners]

Serializers are documented at

pino/docs/api.md

Lines 460 to 474 in eff2b30

	#### `serializers` (Object)
	Default: `{err: pino.stdSerializers.err}`
	An object containing functions for custom serialization of objects.
	These functions should return an JSONifiable object and they
	should never throw. When logging an object, each top-level property
	matching the exact key of a serializer will be serialized using the defined serializer.
	The serializers are applied when a property in the logged object matches a property
	in the serializers. The only exception is the `err` serializer as it is also applied in case
	the object is an instance of `Error`, e.g. `logger.info(new Error('kaboom'))`.
	See `errorKey` option to change `err` namespace.
	* See [pino.stdSerializers](#pino-stdserializers)