Improved docs for LightningModule

[awaelchli]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests? (not needed)
• If you made a notable change (that affects users), did you update the CHANGELOG?

What does this PR do?

Most of the changes are

• Internal links (classes, methods, parameters) for easier navigation of the docs.
• Indentation changes (that looked wrong in the html or broke layout)
• Standardization of certain formatting, like notes, syntax highlighting etc.

Sorry for so many lines changed :(
Let me know if it's too much or if you think some changes are not needed/wanted :)

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #1389 into master will not change coverage by %.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1389   +/-   ##
======================================
  Coverage      92%     92%           
======================================
  Files          63      63           
  Lines        3323    3323           
======================================
  Hits         3051    3051           
  Misses        272     272