runtime(help): Updated documentation on editorconfig

runtime(help): Updated documentation on editorconfig

Add a small section about the distributed Editorconfig plugin at :h
usr_05.txt just below the matchit plugin.  While editing that help
document, also add a bit of more documentation about standard plugins
and local help file additions.

Regenerate $VIMRUNTIME/doc/tags file with all the new tags from the rust
runtime files.

While at it, update the Editorconfig help page (and re-generate the
helptags file).

closes: #13078

Signed-off-by: Christian Brabandt <cb@256bit.org>