Rename "X guidelines" docs to "Writing X"

The meanings of "Client library guidelines" and "Exporter guidelines" are not
immediately clear. For example, it could mean the usage of client
libraries or exporters. I think "Writing client libraries" and "Writing
exporters" makes this clearer.

content/docs/instrumenting/clientlibs.md

@@ -35,7 +35,7 @@ dependencies, you may also implement one of the supported [exposition
 formats](/docs/instrumenting/exposition_formats/) yourself to expose metrics.
 
 When implementing a new Prometheus client library, please follow the
-[Prometheus Client Library Guidelines](/docs/instrumenting/clientlib_guidelines).
+[guidelines on writing client libraries](/docs/instrumenting/writing_clientlibs).
 Note that this document is still a work in progress. Please also consider
 consulting the [development mailing list](https://groups.google.com/forum/#!forum/prometheus-developers).
 We are happy to give advice on how to make your library as useful and

content/docs/instrumenting/exporters.md

@@ -76,7 +76,7 @@ hosted outside of the Prometheus GitHub organization.
    * [Varnish exporter](https://github.com/jonnenauha/prometheus_varnish_exporter)
 
 When implementing a new Prometheus exporter, please follow the
-[Prometheus Exporter Guidelines](/docs/instrumenting/exporter_guidelines)
+[guidelines on writing exporters](/docs/instrumenting/writing_exporters)
 Please also consider consulting the [development mailing
 list](https://groups.google.com/forum/#!forum/prometheus-developers).  We are
 happy to give advice on how to make your exporter as useful and consistent as

content/docs/instrumenting/clientlib_guidelines.md → content/docs/instrumenting/writing_clientlibs.md

 ---
-title: Client library guidelines
+title: Writing client libraries
 sort_rank: 2
 ---
 
-# Client library guidelines
+# Writing client libraries
 
 This document covers what functionality and API Prometheus client libraries
 should offer, with the aim of consistency across libraries, making the easy use

content/docs/instrumenting/exporter_guidelines.md → content/docs/instrumenting/writing_exporters.md

 ---
-title: Exporter guidelines
+title: Writing exporters
 sort_rank: 5
 ---
 
-# Exporter guidelines
+# Writing exporters
 
 When directly instrumenting your own code, the general rules of how to
 instrument code with a Prometheus client library can be followed quite