Add text to new _templ_plugins section

Replace links to ‘writing-plugin’ by internal links to ‘templ_plugins’.
Add small paragraph at the end of _templ_plugins to mention ‘inline’
plugin and the ‘Writing Plugins’ page.
Merge branch 'doc_plugins_templ_funcs' of github.com:KraYmer/beets into doc_plugins_templ_funcs

Conflicts:
	docs/reference/pathformat.rst