documentation: migrate ComponentReference to markdown

This enables us to use toctrees, which informs Sphinx on the global
documentation structure, and removes warnings about not-included
documents.

The old ComponentReference.pod was kept, in order to not break the old
HTML output, but the new Markdown file is used as source for the
generated markdown files in doc/

modules/database/src/std/ComponentReference.md

@@ -0,0 +1,96 @@
+# IOC Component Reference
+
+This document provides reference information about the record types,
+menus, link types and channel filters included with EPICS Base.
+
+## Introduction and IOC Concepts
+
+- [Introduction to EPICS](https://docs.epics-controls.org/en/latest/getting-started/EPICS_Intro.html)
+- [Process Database Concepts](https://docs.epics-controls.org/en/latest/process-database/EPICS_Process_Database_Concepts.html)
+
+## Record Type Definitions
+
+These sections describe common aspects of the record types:
+
+```{toctree}
+:titlesonly:
+
+dbCommonRecord
+dbCommonInput
+dbCommonOutput
+```
+
+These are the record types supplied with EPICS Base:
+
+```{toctree}
+:titlesonly:
+
+aaiRecord
+aaoRecord
+aiRecord
+aoRecord
+aSubRecord
+biRecord
+boRecord
+calcoutRecord
+calcRecord
+compressRecord
+dfanoutRecord
+eventRecord
+fanoutRecord
+histogramRecord
+int64inRecord
+int64outRecord
+longinRecord
+longoutRecord
+lsiRecord
+lsoRecord
+mbbiDirectRecord
+mbbiRecord
+mbboDirectRecord
+mbboRecord
+permissiveRecord
+printfRecord
+selRecord
+seqRecord
+stateRecord
+stringinRecord
+stringoutRecord
+subArrayRecord
+subRecord
+waveformRecord
+```
+
+## Menu Definitions
+
+Menu field choices are documented with the record type that defines them, or
+here for the global menus that are used by multiple record types:
+
+```{toctree}
+:titlesonly:
+
+menuAlarmSevr
+menuAlarmStat
+menuConvert
+menuFtype
+menuIvoa
+menuOmsl
+menuPini
+menuPost
+menuPriority
+menuScan
+menuSimm
+menuYesNo
+```
+
+## Other Components
+
+EPICS Base also comes with extensible sets of server Channel Filters and IOC
+Database Link types, which are documented here:
+
+```{toctree}
+:titlesonly:
+
+filters
+links
+```