Standard installation instruction visibility

[AntonNI8]

Context:
The standard (default, preferred) installation instructions in the pennylane-lightning docs are easy to miss.
This could discourage users if they think they need to go through a more involved installation process, ie. installing from source.

Description of the Change:
Added a heading for standard installation instructions.

Benefits:
Easier to notice the most simple/straightforward installation method for lightning devices.

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit .github/CHANGELOG.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.70%. Comparing base (4c8d840) to head (6d8b6a8).
Report is 1 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (4c8d840) and HEAD (6d8b6a8). Click for more details.

HEAD has 27 uploads less than BASE

Flag	BASE (4c8d840)	HEAD (6d8b6a8)
	33	6

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #979       +/-   ##
===========================================
- Coverage   97.72%   28.70%   -69.03%     
===========================================
  Files         228       28      -200     
  Lines       36378     2505    -33873     
===========================================
- Hits        35550      719    -34831     
- Misses        828     1786      +958     

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[DSGuala]

Looks good to me. Approved pending

1. Rebase to master
2. Making sure there aren't any merge conflicts
3. All tests pass

[AntonNI8]

@maliasadi this PR should be good to go now, pending testing + your review

[maliasadi]

Thanks @AntonNI8! Feel free to merge the PR after updating the changelog 🎉