doc: [~] fix codecov badges & add release and first issues badges

[eruizalo]

Description

Previously codecov badges were wrong (flag not working), now it works and some other badges has been added.

Related Issue

N/A

How Has This Been Tested?

• No tests are needed, only doc modifications

[codecov]

Codecov Report

Merging #246 (e3f938f) into main (aa2494c) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head e3f938f differs from pull request most recent head 9bee67e. Consider uploading reports for the commit 9bee67e to get more accurate results

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
- Coverage   97.29%   97.28%   -0.01%     
==========================================
  Files          58       58              
  Lines        1107     1028      -79     
  Branches       10       12       +2     
==========================================
- Hits         1077     1000      -77     
+ Misses         30       28       -2     

Flag	Coverage Δ
spark-2.4.x	93.91% <ø> (+0.51%)	⬆️
spark-3.0.x	96.51% <ø> (-0.07%)	⬇️
spark-3.1.x	97.40% <ø> (-0.01%)	⬇️
spark-3.2.x	97.40% <ø> (-0.01%)	⬇️
spark-3.3.x	97.40% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
core/src/main/scala/doric/types/SparkType.scala	95.38% <0.00%> (-0.62%)	⬇️
core/src/main/scala/doric/sem/Errors.scala	92.31% <0.00%> (ø)
core/src/main/scala/doric/syntax/DStructs.scala	100.00% <0.00%> (ø)
core/src/main/scala/doric/types/NumericType.scala	100.00% <0.00%> (ø)
...c/main/scala/doric/syntax/LiteralConversions.scala	100.00% <0.00%> (ø)
.../src/main/scala/doric/types/LiteralSparkType.scala	100.00% <0.00%> (+1.35%)	⬆️
core/src/main/scala/doric/DoricColumn.scala	71.43% <0.00%> (+4.76%)	⬆️

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