Revamp documentation

[juarezr]

This PR has the objective of improving documentation.

Changes

1. Modernized frontpages
2. Split installation page in docs

Checklist

Use this checklist for assuring the quality of pull requests that include new code and or make changes to existing code.

• [ ] Source Code rules apply:
  • [ ] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behaviour
  • [ ] All changes are documented in docs/changes.rst
• [X] Versioning and history tracking rules apply:
  • [X] Using atomic commits when possible
  • [X] Commits are reversible when possible
  • [X] There is no incomplete changes in the pull request
  • [X] There is no accidental garbage added in source code
• [X] Testing rules apply:
  • [X] Tested locally using tox / pytest
  • [ ] Automated testing passes (see CI)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [X] State of these changes is:
  • [ ] Just a proof of concept
  • [ ] Work in progress / Further changes needed
  • [X] Ready to review
  • [X] Ready to merge

[coveralls]

Coverage remained the same at 91.759% when pulling 998e802b26b9d5f41ef5f146a2d63c83f87b1435 on juarezr:revamp-docs into 9215549d9351139b13c5b275515d0cc752ee72b8 on petl-developers:master.