Update index.md

[sallybracegirdle]

Edits to the manual homepage to make it less salesy and more focused on guidance.

Please check the following:

• [ ] Has the code POD documentation been added or updated?
• [ ] Whether this PR should include changes to any public documentation, or the FAQ;
• [ ] All cobrand-specific commits start their commit message with the cobrand in square brackets;
• [ ] Is new functionality tested? CodeCov will warn you about the diff coverage, but won’t complain about e.g. new files;
• [ ] Will cobrand-specific changes require additional work to ensure consistent behaviour on www.fixmystreet.com?
• [ ] Are the changes tested for accessibility?
• [ ] Have you updated the changelog? If this is not necessary, put square brackets around this: skip changelog

Please check the contributing docs, and describe your pull request here. Screenshots or GIF animations (using e.g. LICEcap) may be helpful.

Please include any issues that are fixed, using "fixes" or "closes" so that they are auto-closed when the PR is merged.

Thanks for contributing!

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.62%. Comparing base (6f8efe0) to head (b2a3337). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5011 +/- ## ======================================= Coverage 82.62% 82.62% ======================================= Files 395 395 Lines 30796 30796 Branches 4883 4883 ======================================= Hits 25446 25446 - Misses 3900 3901 +1 + Partials 1450 1449 -1 ```

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