move templates from rakyll/statik to go:embed

There is no need for the rakyll/statik package starting with Go 1.16,
which provides us with tools for embedding files
without third-party libraries.

site/src/docs/configuration/email/index.md

@@ -190,7 +190,7 @@ After you set `SMTP_` variables, you can allow email authentication by setting t
 
 ## HTML templates for emails and error messages
 
-Remark42 uses golang templates for email templating. Templates are located in `backend/templates` and embedded into binary by statik
+Remark42 uses golang templates for email templating. Templates are located in `backend/app/templates/static` and embedded into binary by `go:embed` [directive](https://pkg.go.dev/embed).
 
 Now we have the following templates:
 

site/src/docs/contributing/backend/index.md

@@ -30,7 +30,7 @@ Run tests in your IDE, and re-run `make rundev` each time you want to see how yo
 
 You have to [install](https://golang.org/doc/install) the latest stable `go` toolchain to run the backend locally.
 
-In order to have working Remark42 installation you need once to copy frontend static files to `./backend/web` directory from `master` docker image, and also copy files from `./templates` to the `./backend` as they are expected to be where application starts:
+In order to have working Remark42 installation you need once to copy frontend static files to `./backend/web` directory from `master` docker image, as it is expected to be where application compiles:
 
 ```shell
 # frontend files
@@ -38,8 +38,6 @@ docker pull umputun/remark42:master
 docker create -ti --name remark42files umputun/remark42:master sh
 docker cp remark42files:/srv/web/ ./backend/
 docker rm -f remark42files
-# template files
-cp ./backend/templates/* ./backend
 # fix frontend files to point to the right URL
 ## Mac version
 find -E ./backend/web -regex '.*\.(html|js|mjs)$' -print -exec sed -i '' "s|{% REMARK_URL %}|http://127.0.0.1:8080|g" {} \;