SUBMARINE-1411. Create 0.8.0 documents

[cdmikechen]

What is this PR for?

Create 0.8.0 documents for 0.8.0 release

What type of PR is it?

Documentation

Todos

• - Add 0.8.0 document
• - Update download page

What is the Jira issue?

https://issues.apache.org/jira/browse/SUBMARINE-1411

How should this be tested?

NA

Screenshots (if appropriate)

Questions:

• Do the license files need updating? No
• Are there breaking changes for older versions? No
• Does this need new documentation? Yes

[codecov]

Codecov Report

Merging #1107 (39c01ef) into master (4ddba98) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1107   +/-   ##
=======================================
  Coverage   66.83%   66.83%           
=======================================
  Files         130      130           
  Lines        6320     6320           
=======================================
  Hits         4224     4224           
  Misses       2096     2096           

Flag	Coverage Δ
python-integration	53.97% <ø> (ø)
python-unit	47.86% <ø> (ø)

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