Add doc comments to clarify usage of File related methods and leading slash handling

[aldas]

Add doc comments to clarify usage of File related methods and leading slash handling

Relates to #2922 (comment)

Example:

package main

import (
	"embed"

	"github.com/labstack/echo/v5"
)

//go:embed dist/*
var efs embed.FS

func main() {
	e := echo.New()
	e.Filesystem = efs

	e.File("/test", "dist/private.txt") // <--- file path must not have a leading slash

	if err := e.Start(":8080"); err != nil {
		e.Logger.Error("failed to start server", "error", err)
	}
}

and

package main

import (
	"os"

	"github.com/labstack/echo/v5"
)

func main() {
	e := echo.New()
	e.Filesystem = os.DirFS("/")

	e.File("/test.jpg", "opt/app/assets/test.jpg") // <--- file path must not have a leading slash

	if err := e.Start(":8080"); err != nil {
		e.Logger.Error("failed to start server", "error", err)
	}
}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.01%. Comparing base (19364e2) to head (3d2e6b3).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2928   +/-   ##
=======================================
  Coverage   93.01%   93.01%           
=======================================
  Files          43       43           
  Lines        4496     4498    +2     
=======================================
+ Hits         4182     4184    +2     
  Misses        196      196           
  Partials      118      118           

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