Change agenda & add speakers

Add agenda

Add openEuler Summit 2021 minisite

Delete banners.

Add a blog.

Update banners.

Add news about version 20.03 LTS SP2 released.

Add 20.03 LTS SP2

1. Compress pictures to improve loading speed. 2.Delete useless pictures

Change the obs source path of videos and pictures from HK to BeiJing.

Add Bisheng JDK and secGear sites in discovery and update url

1.Add projects minisites 2.Replace banners.

Add reference links.

Update privacy policy.

1.Add sig landscape 2.Update privacy policy.

1.Update openEuler 21.03 white paper 2.add google analytics script

1.replace global font-family 2.update cve page.

Update dev day page.

Update openEuler developer day page.

Bug fix and content update.